Speed of Fronts of the Reaction-Diffusion Equation.

We study the speed of propagation of fronts for the scalar reaction-diffusion equation ut › uxx 1 fsud with fs0d › fs1d › 0. We give a new integral variational principle for the speed of fronts joining the state u › 1 to u › 0. No assumptions are made on the reaction term fsud other than those needed to guarantee the existence of the front. Therefore our results apply to the classical case f . 0 in s0, 1d, to the bistable case, and to cases in which f has more than one internal zero in s0, 1d. [S0031-9007(96)00796-X]