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  • Mathematics  (84)
  • Energy, Environment Protection, Nuclear Power Engineering
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  • 1
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    Order 11 (1994), S. 257-279 
    ISSN: 1572-9273
    Keywords: 06A06 ; 06D05 ; 68Q15 ; Partially ordered set ; Frattini sublattice ; NP-complete
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We show that it is a NP-complete problem to decide whether a finite poset arises as the (Birkhoff) dual of the Frattini sublattice of some finite distributive lattice.
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  • 2
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    Order 11 (1994), S. 11-14 
    ISSN: 1572-9273
    Keywords: 06A06 ; Fixed point property ; retract ; product
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We prove that if the finite ordered setsP andX have the fixed point property then so too doesP×X.
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  • 3
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    Order 11 (1994), S. 47-60 
    ISSN: 1572-9273
    Keywords: 06A06 ; Treewidth ; pathwidth ; dimension ; interval dimension ; cocomparability graphs ; approximation algorithms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We show that the pathwidth of a cocomparability graphG equals its treewidth. The proof is based on a new notion, calledinterval width, for a partial orderP, which is the smallest width of an interval order contained inP, and which is shown to be equal to the treewidth of its cocomparability graph. We observe also that determining any of these parameters isNP-hard and we establish some connections between classical dimension notions of partial orders and interval width. In fact we develop approximation algorithms for interval width ofP whose performance ratios depend on the dimension and interval dimension ofP, respectively.
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  • 4
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    Order 11 (1994), S. 281-305 
    ISSN: 1572-9273
    Keywords: 06A06 ; 06D05 ; Series-parallel poset ; order-reversing map ; distributive lattice ; Ockham algebra
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract A study is undertaken of order-reversing maps on series-parallel posets and structural characterisations are obtained of various subclasses of such ordered sets. The results are applied to complete the authors' earlier investigation of classes $$\mathcal{A}^{rel} $$ of finite relate $$\mathcal{A}$$ lattices, where $$\mathcal{A}$$ is a variety of Ockham lattices and the distributive lattice duals of the algebras in $$\mathcal{A}^{rel} $$ are required to be series-parallel.
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  • 5
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    Order 11 (1994), S. 353-359 
    ISSN: 1572-9273
    Keywords: 06A06 ; Fixed point property ; retractable element ; contractible element ; product ; chain completeness
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract LetP, Q be ordered sets and leta∈P. IfP \ {a} is a retract ofP and setsP and {x∈P:x〉p} (or its dual) have the fixed point property then, for each chain complete setP,P×Q has the fixed point property if and only if (P\{a})×Q has this property. This establishes the fixed point property for some products of ordered sets which are beyond the reach of all known product theorems.
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  • 6
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    Order 11 (1994), S. 169-193 
    ISSN: 1572-9273
    Keywords: 06A06 ; Poset ; crossing number ; product ; lexicographical sum ; Boolean lattice
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The purpose of this paper is to investigate some properties of the crossing number χ(P) of a posetP. We first study the crossing numbers of the product and the lexicographical sum of posets. The results are similar to the dimensions of these posets. Then we consider the problem of what happens to the crossing number when a point is taken away from a poset. We show that ifP is a poset such that χ∈P and χ(P−χ)⩾1, then 1/2 χ(P)⩽χ(P−χ)⩽χ(P). We don't know yet how to improve the lower bound. We also determine the crossing numbers of some subposets of the Boolean latticeB n which consist of some specified ranks. Finally we show that Ψ n is crossing critical where Ψ n is the subposet ofB n which is restricted to rank 1, rankn−1 and middle rank(s). Some open problems are raised at the end of this paper.
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  • 7
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    Order 11 (1994), S. 1-9 
    ISSN: 1572-9273
    Keywords: 06A06 ; Ordered set ; retract ; core
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Theretracts (idempotent, isotone self-maps) of an ordered set are naturally ordered as functions. In this note we characterize the possible ways that one retract can cover another one. This gives some insight into the structure of the ordered set of retracts and leads to a natural generalization of the core of an ordered set.
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  • 8
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    BIT 34 (1994), S. 337-345 
    ISSN: 1572-9125
    Keywords: 65L05 ; 70H99 ; symplectic integration ; Hamiltonian systems ; quadratic invariants
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We show that the theorems of Sanz-Serna and Eirola and Sanz-Serna concerning the symplecticity of Runge-Kutta and Linear Multistep methods, respectively, follow from the fact that these methods preserve quadratic integral invariants and are closed under differentiation and restriction to closed subsystems.
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  • 9
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    BIT 34 (1994), S. 25-32 
    ISSN: 1572-9125
    Keywords: 65L05 ; general linear method ; diagonally implicit method ; DIMSIM ; order conditions ; stability polynomial
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The special case of diagonally-implicit multistage integration methods is considered in which the order, the stage order, the number of values passed between steps and the number of stages in a step, all coincide. It is shown that a similarity transformation can be applied to the matrices characterizing the method so as to simplify the expression for the stability polynomial and thus aid in the search for methods with acceptable stability.
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  • 10
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    BIT 34 (1994), S. 88-98 
    ISSN: 1572-9125
    Keywords: Runge-Kutta ; absolute stability ; Floquet ; equilibrium ; steady state ; 65L05
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract This work examines the stability of explicit Runge-Kutta methods applied to a certain linear ordinary differential equation with periodic coefficients. On this problem naïve use of the eigenvalues of the Jacobian results in misleading conclusions about stable behaviour. It is shown, however, that a valid analogue of the classical absolute stability theory can be developed. Further, using a suitable generalisation of the equilibrium theory of Hall [ACM Trans. on Math. Soft. 11 (1985), pp. 289–301], accurate predictions are made about the performance of modern, adaptive algorithms.
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  • 11
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    BIT 34 (1994), S. 215-227 
    ISSN: 1572-9125
    Keywords: 65L05 ; 65L20 ; Parallel methods ; ordinary differential equations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract This paper concerns parallel frontal predictor-corrector methods. Order and stability of these methods are investigated, when the corrector is solved both by the fixed point iteration method and by the Newton method.
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  • 12
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    BIT 34 (1994), S. 262-267 
    ISSN: 1572-9125
    Keywords: 65L05 ; 65L20 ; Runge-Kutta methods ; A-stability ; delay differential equations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract A natural Runge-Kutta method is a special type of Runge-Kutta method for delay differential equations (DDEs); it is known that any one-step collocation method is equivalent to one of such methods. In this paper, we consider a linear constant-coefficient system of DDEs with a constant delay, and discuss the application of natural Runge-Kutta methods to the system. We show that anA-stable method preserves the asymptotic stability property of the analytical solutions of the system.
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  • 13
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    BIT 34 (1994), S. 455-483 
    ISSN: 1572-9125
    Keywords: 65L05 ; Multibody dynamics ; automatic modeling ; constraints ; invariants ; descriptor form ; state space form ; Runge-Kutta methods ; multistep methods ; one-leg collocation methods ; DAE ; transferred ODE ; velocity stabilization ; projection ; quasi-stabilization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract An application of some one- and multistep methods for the numerical integration ofconstrained mechanical systems with and without stabilization is demonstrated to compare their efficiency. In particular, the use of (high order) multistep methods is considered. Choosingsupernumerary coordinates, constrained mechanical systems are formulated in descriptor form (DAE). For 2D systems at least, an assembling technique methodically different from, but equivalent to, the Lagrange formalism is presented for the computer generation of the equations of motion with constant mass matrix and quadratic constraints. This modeling, which can be generalized to 3D multibody systems, is applied on two non-stiff test problems. An example of the automatic modeling and simulation is given by a seven body mechanism. Computer programs are presented. By index reduction, the DAE model is transferred to an equivalent ODE representation whose unstable numerical solution is stabilized byprojection onto the constraint manifold. A perturbation analysis shows thatvelocity stabilization is the most efficient projection with regard to improvement of the numerical integration. How frequently the numerical solution of the transferred ODE should be stabilized is discussed. For a class of multistep methods, a strategy of stabilizing at certain time steps and performing a much less demandingquasi-stabilization at the others is suggested, especially for high order methods. With these high order methods, stabilization is not even necessary for the second test problem.
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  • 14
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    BIT 34 (1994), S. 325-336 
    ISSN: 1572-9125
    Keywords: 65L05 ; 34K99 ; Delay differential equations ; dynamic iteration ; waveform relaxation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract In this paper we study dynamic iteration techniques for systems of nonlinear delay differential equations. After pointing out a close connection to the ‘truncated infinite embedding’, as proposed by Feldstein, Iserles, and Levin, we give a proof of the superlinear convergence of the simple dynamic iteration scheme. Then we propose a more general scheme that in addition allows for a decoupling of the equations into disjoint subsystems, just like what we are used to from dynamic iteration schemes for ODEs. This scheme is also shown to converge superlinearly.
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  • 15
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    BIT 34 (1994), S. 361-366 
    ISSN: 1572-9125
    Keywords: 65L05 ; Differential-algebraic systems ; Runge-Kutta methods
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Implicit Runge-Kutta (IRK) methods and projected IRK methods for the solution of semiexplicit index-2 systems of differential algebraic systems (DAEs) have been proposed by several authors. In this paper we prove that if a method satisfiesBA+A t B−bb t =0, it conserves quadratic invariants of DAEs.
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  • 16
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    Order 10 (1993), S. 1-16 
    ISSN: 1572-9273
    Keywords: 06A06 ; Planar graph ; triangle-free ; upward drawing
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract A planar ordered set has a triangle-free, planar covering graph; on the other hand, there are nonplanar ordered sets whose covering graphs are planar. We show thatevery triangle-free planar graph has a planar upward drawing. This planar upward drawing can be constructed in time, polynomial in the number of vertices. Our results shed light on the apparently difficult problem, of long-standing, whether there is aneffective planarity-testing procedure for an ordered set.
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  • 17
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    Order 10 (1993), S. 37-54 
    ISSN: 1572-9273
    Keywords: 06A06 ; Ordered set ; cutset ; 2-cutset property ; chain ; antichain ; width ; linear extension ; dimension
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract An ordered setP is said to have 2-cutset property if, for every elementx ofP, there is a setS of elements ofP which are noncomparable tox, with |S|⩽2, such that every maximal chain inP meets {x}∪S. We consider the following question: Does there exist ordered sets with the 2-cutset property which have arbitrarily large dimension? We answer the question in the negative by establishing the following two results.Theorem: There are positive integersc andd such that every ordered setP with the 2-cutset property can be represented asP=X∪Y, whereX is an ordinal sum of intervals ofP having dimension ⩽d, andY is a subset ofP having width ⩽c. Corollary: There is a positive integern such that every ordered set with the 2-cutset property has dimension ⩽n.
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  • 18
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    Order 10 (1993), S. 227-237 
    ISSN: 1572-9273
    Keywords: 06A06 ; 52A37 ; Causal order ; spherical (containment) order ; Minkowski dimension
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The recent work on circle orders generalizes to higher dimensional spheres. As spherical containment is equivalent to causal precedence in Minkowski space, we define the Minkowski dimension of a poset to be the dimension of the minimal Minkowski space into which the poset can be embedded; this isd if the poset can be represented by containment with spheresS d−2 and of no lower dimension. Comparing this dimension with the standard dimension of partial orders we prove that they are identical in dimension two but not in higher dimensions, while their irreducible configurations are the same in dimensions two and three. Moreover, we show that there are irreducible configurations for arbitrarily large Minkowski dimension, thus providing a lower bound for the Minkowski dimension of partial orders.
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  • 19
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    Order 10 (1993), S. 31-36 
    ISSN: 1572-9273
    Keywords: 06A06 ; Priority queue ; binary sequence ; enumeration
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract A priority queue transforms an input sequence σ into an output sequence τ which is a re-ordering of the sequence σ. The setR of all such related pairs is studied in the case that σ is a binary sequence. It is proved thatR is a partial order and that ¦R¦=c n+1, the (n+1)th Catalan number. An efficient (O(n 2)) algorithm is given for computing the number of outputs achievable from a given input.
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  • 20
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    Order 10 (1993), S. 55-63 
    ISSN: 1572-9273
    Keywords: 06A06 ; Poset ; PT-order ; chain complete ; retract ; fixed-point ; core
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract It is well known that dismantling a finite posetP leads to a retract, called the core ofP, which has the fixed-point property if and only ifP itself has this property. The PT-order, or passing through order, of a posetP is the quasi order ⊴ defined onP so thata⊴b holds if and only if every maximal chain ofP which passes througha also passes throughb. This leads to a generalization of the dismantling procedure which works for arbitrary chain complete posets which have no infinite antichain. We prove that such a poset also has a finite core, i.e. a finite retract which reflects the fixed-point property forP.
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  • 21
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    Order 10 (1993), S. 239-252 
    ISSN: 1572-9273
    Keywords: 05C45 ; 06A05 ; 06A06 ; Antimatroid ; basic word graph ; combinatorial Gray code ; Hamiltonicity ; join-distributive lattice
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We show three main results concerning Hamiltonicity of graphs derived from antimatroids. These results provide Gray codes for the feasible sets and basic words of antimatroids. For antimatroid (E, $$\mathcal{F}$$ ), letJ( $$\mathcal{F}$$ ) denote the graph whose vertices are the sets of $$\mathcal{F}$$ , where two vertices are adjacent if the corresponding sets differ by one element. DefineJ( $$\mathcal{F}$$ ;k) to be the subgraph ofJ( $$\mathcal{F}$$ )2 induced by the sets in $$\mathcal{F}$$ with exactlyk elements. Both graphsJ( $$\mathcal{F}$$ ) andJ( $$\mathcal{F}$$ ;k) are connected, and the former is bipartite. We show that there is a Hamiltonian cycle inJ( $$\mathcal{F}$$ )×K 2. As a consequence, the ideals of any poset % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWefv3ySLgznf% gDOfdaryqr1ngBPrginfgDObYtUvgaiuaacqWFpepuaaa!414C!\[\mathcal{P}\] may be listed in such a way that successive ideals differ by at most two elements. We also show thatJ( $$\mathcal{F}$$ ;k) has a Hamilton path if (E, $$\mathcal{F}$$ ) is the poset antimatroid of a series-parallel poset. Similarly, we show thatG( $$\mathcal{L}$$ )×K 2 is Hamiltonian, whereG( $$\mathcal{L}$$ ) is the “basic word graph” of a language antimatroid (E, $$\mathcal{L}$$ ). This result was known previously for poset antimatroids.
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  • 22
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    Order 10 (1993), S. 329-347 
    ISSN: 1572-9273
    Keywords: 06A06 ; Fixed point property ; irreducible point ; dismantable poset ; retractable point
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We introduce retractable points and show how this notion provides the key for a classification of all sets with 11 elements that have the fixed point property.
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  • 23
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    Order 10 (1993), S. 153-160 
    ISSN: 1572-9273
    Keywords: 06A06 ; Tree ; dimension
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    Topics: Mathematics
    Notes: Abstract We call an ordered set (X, ⩽) a tree if no pair of incomparable elements ofX has an upper bound. It is shown that there is a natural way to associate a tree (T, ⩽) with any ordered set (X, ⩽), and (T, ⩽) can be characterized by a universal property. We define the tree dimensiontd(X, ⩽) of an ordered set as the minimal number of extensions of (X, ⩽) which are trees such that the given order is the intersection of those tree orders. We give characterizations of the tree dimension, relations between dimension and tree dimension, and removal theorems.
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  • 24
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    Order 10 (1993), S. 143-145 
    ISSN: 1572-9273
    Keywords: 06A06 ; Sphere containment order ; circle containment order
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Using the ideas of Scheinerman and Wierman [1] and of Hurlbert [2] we give a very short proof that the infinite order [2]×[3]×ω cannot be represented by containment of Euclidean balls in ad-dimensional space for anyd. Also we give representations of the orders [2]×[2]×ω and [3]×[3]×[3] by containment of circles in the plane.
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  • 25
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    Order 10 (1993), S. 183-197 
    ISSN: 1572-9273
    Keywords: 06A06 ; 06A07 ; Dimension ; Boolean algebra ; atomic amalgam
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    Topics: Mathematics
    Notes: Abstract The main aim of this paper is the calculation of the dimension of certain atomic amalgams. These consist of finite Boolean algebras (blocks) pasted together in such a way that a pair of blocks intersects either trivially in the bounds, or the intersection consists of the bounds, an atom, and its complement.
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  • 26
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    Order 10 (1993), S. 297-303 
    ISSN: 1572-9273
    Keywords: 06A06 ; 68Q25 ; Diagram ; orientation ; complexity
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    Topics: Mathematics
    Notes: Abstract In 1987, Nešetřil and Rödl [4] claimed to have proved that the problem of finding whether a given graphG can be oriented as the diagram of a partial order is NP-complete. A flaw was discovered in their proof by Thostrup [11]. Nešetřil and Rödl [5] have since corrected the proof, but the new version is rather complex. We give a simpler and more elementary proof, using a completely different approach.
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  • 27
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    Order 10 (1993), S. 349-361 
    ISSN: 1572-9273
    Keywords: 06A06 ; Chain complete poset ; fixed point property ; perfect sequence ; core
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    Topics: Mathematics
    Notes: Abstract Like dismantling for finite posets, a perfect sequence Π = 〈P ξ :ξ≤λ〉 of a chain complete posetP represents a canonical procedure to produce a coreP λ. It has been proved that if the posetP contains no infinite antichain then this coreP λ is a retract ofP andP has the fixed point property iffP λ has this property. In this paper the condition of having no infinite antichain is replaced by a weaker one. We show that the same conclusion holds under the assumption thatP does not contain a one-way infinite fence or a tower.
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  • 28
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    BIT 33 (1993), S. 285-303 
    ISSN: 1572-9125
    Keywords: 65L05 ; stiffness ; stability ; pseudospectra
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    Topics: Mathematics
    Notes: Abstract It is argued that even for a linear system of ODEs with constant coefficients, stiffness cannot properly be characterized in terms of the eigenvalues of the Jacobian, because stiffness is a transient phenomenon whereas the significance of eigenvalues is asymptotic. Recent theory from the numerical solution of PDEs is adapted to show that a more appropriate characterization can be based upon pseudospectra instead of spectra. Numerical experiments with an adaptive ODE solver illustrate these findings.
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  • 29
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    BIT 33 (1993), S. 452-472 
    ISSN: 1572-9125
    Keywords: AMS(MOS) ; 65L05 ; 65L07 ; General linear method ; order conditions ; stability analysis
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    Topics: Mathematics
    Notes: Abstract We investigate some classes of general linear methods withs internal andr external approximations, with stage orderq and orderp, adjacent to the class withs=r=q=p considered by Butcher. We demonstrate that interesting methods exist also ifs+1=r=q, p=q orq+1,s=r+1=q, p=q orq+1, ands=r=q, p=q+1. Examples of such methods are constructed with stability function matching theA-acceptable generalized Padé approximations to the exponential function.
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  • 30
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    BIT 33 (1993), S. 434-451 
    ISSN: 1572-9125
    Keywords: 34A50 ; 65L05 ; 65L20 ; Ordinary Differential Equations ; Initial Value Problems ; Numerical Methods ; Stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The stability properties of three particular boundary value methods (BVMs) for the solution of initial value problems are considered. Our attention is focused on the BVMs based on the midpoint rule, on the Simpson method and on an Adams method of order 3. We investigate their BV-stability regions by considering the scalar test problem and constant stepsize. The study of the conditioning of the coefficient matrix of the discrete problem is extended to the case of variable stepsize and block ODE problems. We also analyse an appropriate choice for the stepsize for stiff problems. Numerical tests are reported to evidentiate the effectiveness of the BVMs and the differences among the BVMs considered.
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  • 31
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    BIT 33 (1993), S. 619-633 
    ISSN: 1572-9125
    Keywords: 65L05 ; Stiff Ordinary Differential Equations ; Diagonally Implicit Runge-Kutta methods
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract In this paper, the optimal order of non-confluent Diagonally Implicit Runge-Kutta (DIRK) methods with non-zero weights is examined. It is shown that the order of aq-stage non-confluent DIRK method with non-zero weights cannot exceedq+1. In particular the optimal order of aq stage non-confluent DIRK method with non-zero weights isq+1 for 1≤q≤5. DIRK methods of orders five and six in four and five stages respectively are constructed. It is further shown that the optimal order of a non-confluentq stage DIRK method with non-zero weights isq, forq≥6.
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  • 32
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    Numerische Mathematik 66 (1993), S. 411-421 
    ISSN: 0945-3245
    Keywords: 65L05
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    Topics: Mathematics
    Notes: Summary Boundary value techniques for the solution of initial value problems of ODEs, despite their apparent higher cost, present some important advantages over initial value methods. Among them, there is the possibility to have greater accuracy, to control the global error, and to have an efficient parallel implementation. In this paper, the same techniques are applied to the solution of linear initial value problems of DAEs. We have considered three term numerical methods (Midpoint, Simpson, and an Adams type method) in order to obtain a block tridiagonal linear system as a discrete problem. Convergence results are stated in the case of constant coefficients, and numerical examples are given on linear time-varying problems.
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    Numerische Mathematik 64 (1993), S. 409-431 
    ISSN: 0945-3245
    Keywords: 65L05
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    Topics: Mathematics
    Notes: Summary For the numerical solution of non-stiff semi-explicit differentialalgebraic equations (DAEs) of index 1 half-explicit Runge-Kutta methods (HERK) are considered that combine an explicit Runge-Kutta method for the differential part with a simplified Newton method for the (approximate) solution of the algebraic part of the DAE. Two principles for the choice of the initial guesses and the number of Newton steps at each stage are given that allow to construct HERK of the same order as the underlying explicit Runge-Kutta method. Numerical tests illustrate the efficiency of these methods.
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    Numerische Mathematik 64 (1993), S. 521-539 
    ISSN: 0945-3245
    Keywords: 65L05 ; 70-08
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    Topics: Mathematics
    Notes: Summary The numerical analysis of multibody system dynamics is based on the equations of motion as differential-algebraic systems. A thorough analysis of the linearized equations and their solution theory leads to an equivalent system of ordinary differential equations which gives deeper insight into the derivation of integration schemes and into the stabilization approaches. The main tool is the Drazin inverse, a generalized matrix inverse, which preserves the eigenvalues. The results are illustrated by a realistic truck model. Finally, the approach is extended to the nonlinear index 2 formulation.
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    Numerische Mathematik 65 (1993), S. 143-175 
    ISSN: 0945-3245
    Keywords: 35A40 ; 35B40 ; 65L05 ; 65L20 ; 58F22
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    Topics: Mathematics
    Notes: Sumamry This article is concerned with the comparison of the dynamic of a partial differential equation and its time discretization. We restrict our attention to the neighborhood of a hyperbolic periodic orbit. We show that the discretization possesses an invariant closed curve near the periodic orbit and that the trajectories of the semigroups defined by the partial differential equations and its approximation are close in a sense to be precised provided that different data are allowed. This answers partly an open problem posed in [4]. Examples of application to dissipative partial equations are provided.
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    Numerische Mathematik 65 (1993), S. 77-94 
    ISSN: 0945-3245
    Keywords: 34A09 ; 65L05 ; 65H10
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    Topics: Mathematics
    Notes: Summary A method has been proposed for numerically solving lower dimensional, nonlinear, higher index differential algebraic equations for which more classical methods such as backward differentiation or implicit Runge-Kutta may not be appropriate. This method is based on solving nonlinear DAE derivative arrays using nonlinear singular least squares methods. The theoretical foundations, generality, and limitations of this approach remain to be determined. This paper carefully examines several key aspects of this approach. The emphasis is on general results rather than specific results based on the structure of various applications.
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    Numerische Mathematik 65 (1993), S. 285-300 
    ISSN: 0945-3245
    Keywords: 65L05 ; 65L20
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    Topics: Mathematics
    Notes: Summary A trajectory problem is an initial value problem where the interest lies in obtaining the curve traced by the solution, rather than in finding the actual correspondence between the values of the parameter and the points on that curve. This paper introduces a family of multi-stage, multi-step numerical methods to integrate trajectory problems whose solution is on a spherical surface. It has been shown that this kind of algorithms has good numerical properties: consistency, stability, convergence and others that are not standard. The latest ones make them a better choice for certain problems.
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    Numerical algorithms 5 (1993), S. 325-337 
    ISSN: 1572-9265
    Keywords: Chebyshev approximation ; differential equations ; initial value problems ; exchange algorithm ; primary 41A50 ; 65D10 ; 65L05
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    Topics: Computer Science , Mathematics
    Notes: Abstract The nonlinear Chebyshev approximation of real-valued data is considered where the approximating functions are generated from the solution of parameter dependent initial value problems in ordinary differential equations. A theory for this process applied to the approximation of continuous functions on a continuum is developed by the authors in [17]. This is briefly described and extended to approximation on a discrete set. A much simplified proof of the local Haar condition is given. Some algorithmic details are described along with numerical examples of best approximations computed by the Exchange algorithm and a Gauss-Newton type method.
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    Order 9 (1992), S. 245-253 
    ISSN: 1572-9273
    Keywords: 06A06 ; Posets ; chains ; antichains ; matchings ; covers
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    Topics: Mathematics
    Notes: Abstract It is proved that if (P⩽) is a poset with no infinite chain and k is a positive integer, then there exist a partition of P into disjoint chains C i and disjoint antichains A 1, A 2, ..., A k, such that each chain C i meets min (k, |C i|) antichains A j. We make a ‘dual’ conjecture, for which the case k=1 is: if (P⩽) is a poset with no infinite antichain, then there exist a partition of P into antichains A i and a chain C meeting all A i. This conjecture is proved when the maximal size of an antichain in P is 2.
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    Order 9 (1992), S. 305-310 
    ISSN: 1572-9273
    Keywords: 06A06 ; Fixed point property ; strong fixed point property
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    Topics: Mathematics
    Notes: Abstract An ordered set which has the fixed point property but not the strong fixed point property is presented
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    Order 9 (1992), S. 15-29 
    ISSN: 1572-9273
    Keywords: 06A06 ; (Partially) ordered set ; order preserving map ; enumeration ; stochastic process ; martingale
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    Topics: Mathematics
    Notes: Abstract Three results are obtained concerning the number of order preserving maps of an n-element partially ordered set to itself. We show that any such ordered set has at least 2 2n/3 order preserving maps (and 2 2 in the case of length one). Precise asymptotic estimates for the numbers of self-maps of crowns and fences are also obtained. In addition, lower bounds for many other infinite families are found and several precise problems are formulated.
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    Order 9 (1992), S. 233-238 
    ISSN: 1572-9273
    Keywords: 06A06 ; 08A40 ; Order ; projective ; ramified ; monotone idempotent operation
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    Topics: Mathematics
    Notes: Abstract We prove a closure property of the class of projective orders without infinite chains, and strengthen Larose's theorem on the equivalence between projectivity and quasiprojectivity for finite orders.
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    Order 9 (1992), S. 239-244 
    ISSN: 1572-9273
    Keywords: 06A06 ; 08A40 ; Ternary discriminator ; dual discriminator ; near-projection ; bounded partial order ; functionally complete ; Rosenberg's completeness theorem
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    Topics: Mathematics
    Notes: Abstract Algebras with an operator that discriminates order are functionally complete.
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    Order 9 (1992), S. 291-302 
    ISSN: 1572-9273
    Keywords: 06A06 ; Partial orders ; algorithms ; N-free orders ; interval orders
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    Topics: Mathematics
    Notes: Abstract We investigate problems related to the set of minimal interval extensions of N-free orders. This leads us to a correspondence between this set for an arbitrary order and a certain set of its maximal N-free reductions. We also get a 1-1-correspondence between the set of linear extensions of an arbitrary order and the set of minimal interval extensions of the linegraph of that order. This has an algorithmic consequence, namely the problem of counting minimal interval extensions of an N-free order is #P-complete. Finally a characterization of all N-free orders with isomorphic root graph is given in terms of their lattice of maximal antichains; the lattices are isomorphic iff the root graphs agree.
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    Order 9 (1992), S. 357-360 
    ISSN: 1572-9273
    Keywords: 06A06 ; 68M20 ; Ordered sets ; width ; maximum suborder
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    Topics: Mathematics
    Notes: Abstract Finding the largest suborder of fixed width in a partially ordered set is an interesting combinatorial problem with applications in combinatorial optimization and scheduling. We present a polynomial time solution for this problem by transforming it into a minimum cost network flow problem in an appropriate auxiliary network.
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    Order 9 (1992), S. 139-158 
    ISSN: 1572-9273
    Keywords: 06A06 ; Dimension ; fractional dimension
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    Notes: Abstract Given a partially ordered setP=(X, ≤), a collection of linear extensions {L 1,L 2,...,L r } is arealizer if, for every incomparable pair of elementsx andy, we havex〈y in someL i (andy〈x in someL j ). For a positive integerk, we call a multiset {L 1,L 2,...,L t } ak-fold realizer if for every incomparable pairx andy we havex〈y in at leastk of theL i 's. Lett(k) be the size of a smallestk-fold realizer ofP; we define thefractional dimension ofP, denoted fdim(P), to be the limit oft(k)/k ask→∞. We prove various results about the fractional dimension of a poset.
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    Order 9 (1992), S. 311-319 
    ISSN: 1572-9273
    Keywords: 06A06 ; (Strong) fixed point property ; lexicographic sum
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    Topics: Mathematics
    Notes: Abstract The following theorem is proved: If Q=L{P t ∣t∈T} is a finite lexicographic sum of posets such that T and all P t have the strong fixed point property then Q has the strong fixed point property. Moreover we show the strong fixed point property for two more classes of posets.
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    Order 9 (1992), S. 5-13 
    ISSN: 1572-9273
    Keywords: 06-04 ; 06A06 ; 90C35 ; Diagram ; partially ordered set ; algorithm ; iteration
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    Topics: Mathematics
    Notes: Abstract An algorithm is presented to draw Hasse-diagrams of partially ordered sets (orders). It uses two heuristic principles to generate ‘good’ pictures for a wide range of orders. These two principles are (i) The total length of all edges of the diagram should be small (with the vertices kept at a minimal distance) and (ii) the vertices are constrained to coincide with the grid points of a given rectangular planar grid. The benefits are quite straightforward sine (i) using less ink means less confusion and (ii) the restriction to grid points tends to keep the number of different slopes small. Since the program was conceived as a readily usable tool (with the emphasis on results rather than on perfection), we are well aware of the fact that it will lend itself easily to improvements in many aspects.
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    Order 9 (1992), S. 321-331 
    ISSN: 1572-9273
    Keywords: 06A06 ; (Partially) ordered set ; PT-order ; chain complete ; retract ; fixed-point property
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    Topics: Mathematics
    Notes: Abstract The PT-order, or passing through order, of a poset P is a quasi order ⊴ defined on P so that a⊴b holds if and only if every maximal chain of P which passes throug a also passes through b. We show that if P is chain complete, then it contains a subset X which has the properties that (i) each element of X is ⊴-maximal, (ii) X is a ⊴-antichain, and (iii) X is ⊴-dominating; we call such a subset a ⊴-good subset of P. A ⊴-good subset is a retract of P and any two ⊴-good subsets are order isomorphic. It is also shown that if P is chain complete, then it has the fixed point property if and only if a ⊴-good subset also has the fixed point property. Since a retract of a chain complete poset is also chain complete, the construction may be iterated transfinitely. This leads to the notion of the “core” of P (a ⊴-good subset of itself) which is the transfinite analogue of the core of a finite poset obtained by dismantling.
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    BIT 32 (1992), S. 535-538 
    ISSN: 1572-9125
    Keywords: 65L05 ; 65D30 ; 65D32
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    Topics: Mathematics
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    BIT 32 (1992), S. 345-349 
    ISSN: 1572-9125
    Keywords: 65L05 ; 70H15 ; 05C05
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    Topics: Mathematics
    Notes: Abstract Implicit Runge-Kutta methods with vanishingM matrix are discussed for preserving the symplectic structure of Hamiltonian systems. The number of the order conditions independent of the number of stages can be reduced considerably for the symplectic IRK method through the analysis utilizing the rooted tree and the corresponding elementary differentials. Butcher's simplifying condition further reduces the number of independent order conditions.
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    BIT 32 (1992), S. 350-366 
    ISSN: 1572-9125
    Keywords: 65L05 ; 65L20 ; 65Y05
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    Topics: Mathematics
    Notes: Abstract For a large class of traditional backward Euler multirate methods we show that stability is preserved when the methods are applied to certain stable (but not necessarily monotonic) non-linear systems. Methods which utilize waveform relaxation sweeps are shown to be stable and converge for certain monotonic systems.
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    Numerische Mathematik 61 (1992), S. 261-264 
    ISSN: 0945-3245
    Keywords: 65L05
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    Topics: Mathematics
    Notes: Summary Kirchgraber derived in 1988 an integration procedure (called the LIPS-code) for long-term prediction of the solutions of equations which are perturbations of systems having only periodic solutions. His basic idea is to use the Poincaré map to define a new system which can be integrated with large step-size; the method is specially successful when the period is close to the unperturbed one. Obviously the size of the perturbation modifies the period and therefore affects the precision of the algorithm. In this paper we propose a double modification of Kirchgraber's code: to use a first-order approximation of the perturbed period instead of the unperturbed one, and a scheme specially designed for integration of orbits instead of the Runge-Kutta method. We show that this new code permits a spectacular improvement in accuracy and computation time.
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    Numerische Mathematik 61 (1992), S. 281-290 
    ISSN: 0945-3245
    Keywords: 65L05
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    Topics: Mathematics
    Notes: Summary We consider the question of whether multistep methods inherit in some sense quadratic first integrals possessed by the differential system being integrated. We also investigate whether, in the integration of Hamiltonian systems, multistep methods conserve the symplectic structure of the phase space.
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    Numerische Mathematik 61 (1992), S. 291-310 
    ISSN: 0945-3245
    Keywords: 65L05 ; 65L10
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    Topics: Mathematics
    Notes: Summary A finite-difference method for the integration of the Korteweg-de Vries equation on irregular grids is analyzed. Under periodic boundary conditions, the method is shown to be supraconvergent in the sense that, though being inconsistent, it is second order convergent. However, such a convergence only takes place on grids with an odd number of points per period. When a grid with an even number of points is used, the inconsistency of the method leads to divergence. Numerical results backing the analysis are presented.
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    Numerische Mathematik 61 (1992), S. 501-521 
    ISSN: 0945-3245
    Keywords: 65L05 ; 65L07 ; 58F05 ; 70-08 ; 70F15 ; 70H15
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    Topics: Mathematics
    Notes: Summary New methods are presented for the numerical integration of ordinary differential equations of the important family of Hamiltonian dynamical systems. These methods preserve the Poincaré invariants and, therefore, mimic relevant qualitative properties of the exact solutions. The methods are based on a Runge-Kutta-type ansatz for the generating function to realize the integration steps by canonical transformations. A fourth-order method is given and its implementation is discussed. Numerical results are presented for the Hénon-Heiles system, which describes the motion of a star in an axisymmetric galaxy.
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    Order 8 (1991), S. 107-113 
    ISSN: 1572-9273
    Keywords: 06A06 ; Order ; genus ; invariance
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    Topics: Mathematics
    Notes: Abstract The genus of any ordered set equals the genus of its covering graph, and, therefore, the genus of an ordered set is a diagram invariant.
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    Order 8 (1991), S. 29-32 
    ISSN: 1572-9273
    Keywords: 06A06 ; (strong) fixed point property
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    Topics: Mathematics
    Notes: Abstract It is shown that every partially ordered set with the fixed point property and with ten or fewer elements actually has the strong fixed point property.
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    Order 8 (1991), S. 341-348 
    ISSN: 1572-9273
    Keywords: 06A06 ; Colored posets ; zigzags ; order varieties ; irreducible posets
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    Topics: Mathematics
    Notes: Abstract In a 1981 paper, Duffus and Rival define an order variety as a class of posets that is closed under the formation of products and retracts. They also introduce the notion of an irreducible poset. In the present paper we define nonextendible colored posets and certain minimal nonextendible colored posets that we call zigzags. We characterize via nonextendible colored posets the order varieties generated by a set of posets. If the generating set contains only finite posets our characterization is via zigzags. By using these theorems we give a characterization of finite irreducible posets. As an application we show that two different finite irreducible posets generate two different order varieties. We also show that there is a poset which has two different representations by irreducible posets. We thereby settle two open problems listed in the Duffus and Rival paper.
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    Order 8 (1991), S. 349-358 
    ISSN: 1572-9273
    Keywords: 06A06 ; Posets ; strict morphisms ; multiplicativity ; Hedetniemi's conjecture
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    Topics: Mathematics
    Notes: Abstract A functionf from the posetP to the posetQ is a strict morphism if for allx, y ∈ P withx〈y we havef(x)〈f(y). If there is such a strict morphism fromP toQ we writeP → Q, otherwise we writeP $$\not \to $$ Q. We say a posetM is multiplicative if for any posetsP, Q withP $$\not \to $$ M andQ $$\not \to $$ M we haveP ×Q $$\not \to $$ M. (Here (p 1,q 1)〈(p 2,q 2) if and only ifp 1〈p 2 andq 1〈q 2.) This paper proves that well-founded trees with height ≤ω are multiplicative posets.
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    Order 8 (1991), S. 247-265 
    ISSN: 1572-9273
    Keywords: 05A15 ; 05A19 ; 06A06 ; 54-04 ; Quasiordered set ; partially ordered set ; topology ; connected ; separation axiom ; antichain ; generating function
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    Topics: Mathematics
    Notes: Abstract A refinement of an algorithm developed by Culberson and Rawlins yields the numbers of all partially ordered sets (posets) with n points and k antichains for n≤11 and all relevant integers k. Using these numbers in connection with certain formulae derived earlier by the first author, one can now compute the numbers of all quasiordered sets, posets, connected posets etc. with n points for n≤14. Using the well-known one-to-one correspondence between finite quasiordered sets and finite topological spaces, one obtains the numbers of finite topological spaces with n points and k open sets for n≤11 and all k, and then the numbers of all topologies on n≤14 points satisfying various degrees of separation and connectedness properties, respectively. The number of (connected) topologies on 14 points exceeds 1023.
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    Order 8 (1991), S. 7-15 
    ISSN: 1572-9273
    Keywords: 06A06 ; Conductance ; isoperimetric inequality ; linear extension ; poset ; uniform generation ; sorting
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    Topics: Mathematics
    Notes: Abstract Let Q be a convex solid in ℝ n , partitioned into two volumes u and v by an area s. We show that s〉min(u,v)/diam Q, and use this inequality to obtain the lower bound n -5/2 on the conductance of order Markov chains, which describe nearly uniform generators of linear extensions for posets of size n. We also discuss an application of the above results to the problem of sorting of posets.
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    Order 8 (1991), S. 41-48 
    ISSN: 1572-9273
    Keywords: 05C15 ; 05C20 ; 06A06 ; Chromatic number ; partially ordered sets ; dimension ; Hasse diagrams
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    Topics: Mathematics
    Notes: Abstract We construct posets of dimension 2 with highly chromatic Hasse diagrams. This solves a previous problem by Nesetril and Trotter.
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    Order 8 (1991), S. 243-246 
    ISSN: 1572-9273
    Keywords: 06A06 ; Poset ; circle order ; dimension
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    Notes: Abstract The result stated in the title is proved in this note. Actually we show that S x N is not a circle order, where S={(1, 1), (1, 2), (1, 3), (2, 1), (2, 3)}. Furthermore this non-circle order is critical in the sense that (S-{x}) x N is a circle order for any x in S.
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    Order 8 (1991), S. 33-40 
    ISSN: 1572-9273
    Keywords: 06A06 ; 08A40 ; Isotone operation ; ordered set ; projective ; ramified
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    Topics: Mathematics
    Notes: Abstract We show that quasiprojectivity and projectivity are equivalent properties for finite ordered sets of more than two elements.
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    Order 8 (1991), S. 175-183 
    ISSN: 1572-9273
    Keywords: 06A06 ; 68Q25 ; 05C85 ; Transitive closure ; N-free partial order ; two dimensional partial order
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    Topics: Mathematics
    Notes: Abstract Most papers dealing with partial orders assume that the input is given either in transitively closed or transitively reduced form. In this paper, we show that it is possible to solve some problems on partial orders in less time than it takes to perform transitive closure or reduction for general graphs. In particular, we present efficient algorithms for recognizing two dimensional partial orders and N-free partial orders when no assumptions are made about the form of the input.
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    Order 8 (1991), S. 225-242 
    ISSN: 1572-9273
    Keywords: 06A06 ; 68C25 ; Partial order ; linear extension ; #P-complete
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    Topics: Mathematics
    Notes: Abstract We survey the problem of counting the number of linear extensions of a partially ordered set. We show that this problem is #P-complete, settling a long-standing open question. This result is contrasted with recent work giving randomized polynomial-time algorithms for estimating the number of linear extensions. One consequence of our main result is that computing the volume of a rational polyhedron is strongly #P-hard. We also show that the closely related problems of determining the average height of an element x of a give poset, and of determining the probability that x lies below y in a random linear extension, are #P-complete.
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    Order 8 (1991), S. 325-329 
    ISSN: 1572-9273
    Keywords: 06A06 ; Fixed point property ; product ; dismantlable poset ; irreducible point
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    Topics: Mathematics
    Notes: Abstract We give a lower bound for the number of orders on a finite set that are not dismantlable and have the fixed point property. To do so we also derive an upper bound for the number of orders on a finite set that are dismantlable.
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    BIT 31 (1991), S. 647-663 
    ISSN: 1572-9125
    Keywords: 65L05 ; Runge-Kutta ; parallelism ; defect ; interpolation
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    Topics: Mathematics
    Notes: Abstract How can small-scale parallelism best be exploited in the solution of nonstiff initial value problems? It is generally accepted that only modest gains inefficiency are possible, and it is often the case that “fast” parallel algorithms have quite crude error control and stepsize selection components. In this paper we consider the possibility of using parallelism to improvereliability andfunctionality rather than efficiency. We present an algorithm that can be used with any explicit Runge-Kutta formula. The basic idea is to take several smaller substeps in parallel with the main step. The substeps provide an interpolation facility that is essentially free, and the error control strategy can then be based on a defect (residual) sample. If the number of processors exceeds (p − 1)/2, wherep is the order of the Runge-Kutta formula, then the interpolant and the error control scheme satisfy very strong reliability conditions. Further, for a given orderp, the asymptotically optimal values for the substep lengths are independent of the problem and formula and hence can be computed a priori. Theoretical comparisons between the parallel algorithm and optimal sequential algorithms at various orders are given. We also report on numerical tests of the reliability and efficiency of the new algorithm, and give some parallel timing statistics from a 4-processor machine.
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    BIT 31 (1991), S. 608-619 
    ISSN: 1572-9125
    Keywords: 65L05 ; error control ; Adams method ; embedded Runga-Kutta
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract We discuss error control for explicit methods when the stepsize is bounded by stability on the imaginary axis. Our main result is a formulation of a condition on the estimator of the local error which prevents the fast components to exceed the prescribed error tolerance. A PECE Adams method of 4th order accuracy is proposed for mildly stiff oscillatory systems. For comparison we also discuss embedded Runga-Kutta methods.
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  • 71
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    BIT 31 (1991), S. 89-101 
    ISSN: 1572-9125
    Keywords: 65L05 ; Numerical analysis ; stiff problems ; Rosenbrock methods
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract In this paper a new technique for avoiding exact Jacobians in ROW methods is proposed. The Jacobiansf' n are substituted by matricesA n satisfying a directional consistency conditionA n f n =f' n f n +O(h). In contrast to generalW-methods this enables us to reduce the number of order conditions and we construct a 2-stage method of order 3 and families of imbedded 4-stage methods of order 4(3). The directional approximation of the Jacobians has been realized via rank-1 updating as known from quasi-Newton methods.
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  • 72
    ISSN: 1572-9125
    Keywords: 65L05 ; stiff convergence ; linear multistep methods ; one-leg methods
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract To prove convergence of numerical methods for stiff initial value problems, stability is needed but also estimates for the local errors which are not affected by stiffness. In this paper global error bounds are derived for one-leg and linear multistep methods applied to classes of arbitrarily stiff, nonlinear initial value problems. It will be shown that under suitable stability assumptions the multistep methods are convergent for stiff problems with the same order of convergence as for nonstiff problems, provided that the stepsize variation is sufficiently regular.
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  • 73
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    BIT 31 (1991), S. 144-159 
    ISSN: 1572-9125
    Keywords: 41A20 ; 65L05 ; Rational approximation ; A-acceptability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract Rational approximations to the exponential function with real, not necessarily distinct poles are studied in this paper. The orthogonality relation is established in order to show that the zeros of the collocation polynomial of the corresponding Runge-Kutta method are all real, simple and positive. It is proven, that approximants with the smallest error constant are the Restricted Padé approximants of Nørsett. Some results concerning acceptability properties are given.
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  • 74
    ISSN: 1572-9125
    Keywords: 65L05 ; Second order periodic initial-value problem ; phase-lag
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract A four-step method with phase-lag of infinite order is developed for the numerical integration of second order initial-value problems. Extensive numerical testing indicates that this new method can be generally more accurate than other four-step methods.
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  • 75
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    BIT 31 (1991), S. 293-313 
    ISSN: 1572-9125
    Keywords: AMS(MOS) 65M10 ; 65L05 ; 47D05 ; Resolvent conditions ; evolution equations ; numerical stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract As many numerical processes for time discretization of evolution equations can be formulated as analytic mappings of the generator, they can be represented in terms of the resolvent. To obtain stability estimates for time discretizations, one therefore would like to carry known estimates on the resolvent back to the time domain. For different types of bounds of the resolvent of a linear operator, bounds for the norm of the powers of the operator and for their sum are given. Under similar bounds for the resolvent of the generator, some new stability bounds for one-step and multistep discretizations of evolution equations are then obtained.
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  • 76
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    BIT 31 (1991), S. 373-374 
    ISSN: 1572-9125
    Keywords: 65L05
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The polynomial associated with the largest disk of stability of anm-stage explict Runge-Kutta method of orderp is unique.
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  • 77
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    Numerische Mathematik 60 (1991), S. 465-476 
    ISSN: 0945-3245
    Keywords: 65L05 ; 65L20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary In this paper the maximum attainable order of a special class of symmetrizers for Gauss methods is studied. In particular, it is shown that a symmetrizer of this type for thes-stage Gauss method can attain order 2s-1 only for 1 ≦s ≦ 3, and that these symmetrizers areL-stable. A classification of the maximum attainable order of symmetrizers for some higher stages is presented. AnL-stable symmetrizer is also shown to exist for each of the methods studied.
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  • 78
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    Numerische Mathematik 60 (1991), S. 145-161 
    ISSN: 0945-3245
    Keywords: 65L05 ; 65F20
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    Topics: Mathematics
    Notes: Summary This paper presents a family of methods for accurate solution of higher index linear variable DAE systems, $$A(t)\dot y(t) + B(t)y(t) = f(t)$$ . These methods use the DAE system and some of its first derivatives as constraints to a least squares problem that corresponds to a Taylor series ofy, or an approximative equality derived from a Pade' approximation of the exponential function. Accuracy results for systems transformable to standard canonical form are given. Advantages, disadvantages, stability properties and implementation of these methods are discussed and two numerical examples are given, where we compare our results with results from more traditional methods.
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  • 79
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    Numerische Mathematik 59 (1991), S. 321-348 
    ISSN: 0945-3245
    Keywords: 65D05 ; 65L05
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    Topics: Mathematics
    Notes: Summary Construction of optimal triangular meshes for controlling the errors in gradient estimation for piecewise linear interpolation of data functions in the plane is discussed. Using an appropriate linear coordinate transformation, rigorously optimal meshes for controlling the error in quadratic data functions are constructed. It is shown that the transformation can be generated as a curvilinear coordinate transformation for anyC data function with nonsingular Hessian matrix. Using this transformation, a construction of nearly optimal meshes for general data functions is described and the error equilibration properties of these meshes discussed. In particular, it is shown that equilibration of errors is not a sufficient condition for optimality. A comparison of meshes generated under several different criteria is made, and their equilibrating properties illustrated.
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  • 80
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    Numerische Mathematik 59 (1991), S. 723-737 
    ISSN: 0945-3245
    Keywords: 65L05
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    Topics: Mathematics
    Notes: Summary In this paper we give conditions for theB-convergence of Rosenbrock type methods when applied to stiff semi-linear systems. The convergence results are extended to stiff nonlinear systems in singular perturbation form. As a special case partitioned methods are considered. A third order method is constructed.
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  • 81
    ISSN: 0945-3245
    Keywords: 65F20 ; 65F25 ; 65L05
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary This paper extends the singular value decomposition to a path of matricesE(t). An analytic singular value decomposition of a path of matricesE(t) is an analytic path of factorizationsE(t)=X(t)S(t)Y(t) T whereX(t) andY(t) are orthogonal andS(t) is diagonal. To maintain differentiability the diagonal entries ofS(t) are allowed to be either positive or negative and to appear in any order. This paper investigates existence and uniqueness of analytic SVD's and develops an algorithm for computing them. We show that a real analytic pathE(t) always admits a real analytic SVD, a full-rank, smooth pathE(t) with distinct singular values admits a smooth SVD. We derive a differential equation for the left factor, develop Euler-like and extrapolated Euler-like numerical methods for approximating an analytic SVD and prove that the Euler-like method converges.
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  • 82
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    BIT 30 (1990), S. 516-528 
    ISSN: 1572-9125
    Keywords: 65L05 ; 65L20 ; 65M10 ; Numerical analysis ; Initial Value Problems ; Explicit Methods ; Stability ; Stability Region ; Circle Condition
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract This paper deals with polynomial approximationsφ(x) to the exponential function exp(x) related to numerical procedures for solving initial value problems. Motivated by stability requirements, we present a numerical study of the largest diskD(ρ)={z ∈ C: |z+ρ|≤ρ} that is contained in the stability regionS(φ)={z ∈C: |φ(z)|≤1}. The radius of this largest disk is denoted byr(φ), the stability radius. On the basis of our numerical study, several conjectures are made concerningr m,p=sup {r(φ):φ εΠ m,p}. HereΠ m, p (1≤p≤m; p, m integers) is the class of all polynomialsφ(x) with real coefficients and degree ≤m for whichφ(x)=exp(x)+O(x p+1) (forx → 0).
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    BIT 30 (1990), S. 700-706 
    ISSN: 1572-9125
    Keywords: 65L05
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Abstract The theory of positive real functions is used to provide bounds for the largest possible disk to be inscribed in the stability region of an explicit Runge-Kutta method. In particular, we show that the closed disk |ξ+r| ≤r can be contained in the stability region of an explicitm-stage Runge-Kutta method of order two if and only ifr ≤m − 1.
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    Numerische Mathematik 58 (1990), S. 479-501 
    ISSN: 0945-3245
    Keywords: AMS(MOS) ; 65L05 ; CR: G1.7
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mathematics
    Notes: Summary The possibility of balancing the iteration and discretization errors in iterative solution of large systems of initial value problems is discussed. The main result answers the question affirmatively by stating that in the convergence process, in a model situation, the decay rate of iteration errors is essentially independent of the step size refinement process.
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