Variational Characterization of the Speed of Propagation of Fronts for   the Nonlinear Diffusion Equation
  We give an integral variational characterization for the speed of fronts of the nonlinear diffusion equation $u_t = u_{xx} + f(u)$ with $f(0)=f(1)=0$, and $f>0$ in $(0,1)$, which permits, in principle, the calculation of the exact speed for arbitrary $f$. 
R. D. Benguria and M. C. Depassier (Facultad de Fisica P. Universidad   Catolica de Chile Casilla 306, Santiago 22, Chile)