We study the propagation of uniformly translating fronts into a linearly unstable state, both analytically and numerically. We introduce a perturbative renormalization group approach to compute the change in the propagation speed when the fronts are perturbed by structural modification of their governing equations. This approach is successful when the fronts are structurally stable, and allows us to select uniquely the (numerical) experimentally observable propagation speed. For convenience and completeness, the structural stability argument is also briefly described. We point out that the solvability condition widely used in studying dynamics of nonequilibrium systems is equivalent to the assumption of physical renormalizability. We also implement a variational principle, due to Hadeler and Rothe [J. Math. Biol. 2, 251 (1975)], which provides a very good upper bound and, in some cases, even exact results on the propagation speeds, and which identifies the transition from ‘‘linear marginal stability’’ to ‘‘nonlinear marginal stability’’ as parameters in the governing equation are varied.

• Received 5 January 1994

DOI:https://doi.org/10.1103/PhysRevE.49.4502