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This paper is concerned with some nonlinear propagation phenomena forreaction-advection-diffusion equations in a periodic framework. It deals withtravelling wave solutions of the equation $$u_t =\nabla\cdot(A(z)\nabla u)+q(z)\cdot\nabla u+ f(z,u), t \in \mathbb{R}, z \in \Omega,$$ propagating witha speed $c.$ In the case of a "combustion" nonlinearity, the speed $c$ existsand it is unique, while the front $u$ is unique up to a translation in $t.$ Wegive a $\min-\max$ and a $\max-\min$ formula for this speed $c.$ On the otherhand, in the case of a "ZFK" or a "KPP" nonlinearity, there exists a minimalspeed of propagation $c^{*}.$ In this situation, we give a $\min-\max$ formulafor $c^{*}.$ Finally, we apply this $\min-\max$ formula to prove a variationalformula involving eigenvalue problems for the minimal speed $c^{*}$ in the"KPP" case.

Min-Max formulae for the speeds of pulsating travelling fronts in periodic excitable media