Global continuation of monotone waves for bistable delayed equations with unimodal nonlinearities
We study the existence of monotone wavefronts for a general family of bistable reaction-diffusion equations with delayed reaction term g. Differently from previous works, we do not assume the monotonicity of g(u, v) with respect to the delayed variable v that does not allow to apply the comparison techniques. Thus our proof is based on a variant of the Hale-Lin functional-analytic approach to heteroclinic solutions of functional differential equations where Lyapunov-Schmidt reduction is done in appropriate weighted spaces of C-2-smooth functions. This method requires a detailed analysis of associated linear differential operators and their formal adjoints. For two different types of v-unimodal functions g(u, v), we establish the existence of a maximal continuous family of bistable monotone wavefronts. Depending on the type of unimodality (equivalently, on the sign of the wave speed), two different scenarios can be observed for the obtained bistable waves: (1) independently on the size of delay, each bistable wavefront is monotone; (2) wavefronts are monotone for moderate values of delays and can oscillate for large delays.