function [ fval ] = F(t,x,u,ux,duxx)
%
% Written by:
% -- 
% John L. Weatherwax                2005-06-15
% 
% email: wax@alum.mit.edu
% 
% Please send comments and especially bug reports to the
% above email address.
% 
%-----

fval = duxx - u^2;