add the sup of a function on a mesh
real uinfty=sup(Th,abs(f));
real uhinfty=sup(uh); 

  Inforce periodic on  adpated  mesh  today
by adding periodic=[[2,y],[4,y],[1,x],[3,x]].

Newton algorithm.
Non Linear Conjugued Gradient.

computation of eigenvalue and eigenvector
automatique scheme for time depend problem:
dt(u) or u_t

implicite:
problem a(u,v, sheme=Euler , dt = 0.1, tinit = 0, tfinal = 10 , previous=[u1]) = 
  int2d( u_t*v + dx(u)*dx(v)+dy(u)*dy(v) ) ;
  
explicit:
problem a(u,v, sheme=thetaScheme(0.5) , dt = 0.1 , tinit = 0  , tfinal = 10 , previous=[u1]) = 
  int2d( u_t*v) + int(dx(u1)*dx(v)+dy(u1)*dy(v)) ;
  







