//    Discontinous Galerlin Method
//   based on paper from 
// Riviere, Beatrice; Wheeler, Mary F.; Girault, Vivette
// title: 
// A priori error estimates for finite element 
// methods based on discontinuous approximation spaces
//  for elliptic problems.
//  SIAM J. Numer. Anal. 39 (2001), no. 3, 902--931 (electronic).
//  ---------------------------------
//  Formulation given by Vivette Girault
//  ------ 
// Author: F. Hecht , december 2003
// -------------------------------
//   nonsymetric bilinear form
//   ------------------------
//  solve $ -\Delta u = f$ on $\Omega$ and $u= g$ on $\Gamma$
macro dn(u) (N.x*dx(u)+N.y*dy(u) ) //  def the normal derivative 

mesh Th = square(10,10); // unite square 
fespace Vh(Th,P2dc);     // Discontinous P2 finite element
fespace Xh(Th,P2);
//  if param = 0 => Vh must be P2 otherwise we need some penalisation  
real pena=0; // a paramater to add penalisation 
varf Ans(u,v)= 
   int2d(Th)(dx(u)*dx(v)+dy(u)*dy(v)  )
 + intalledges(Th)(//  loop on all  edge of all triangle 
       // the edge are see nTonEdge times so we / nTonEdge
       // remark: nTonEdge =1 on border edge and =2 on internal 
       // we are in a triange th normal is the exterior normal
       // def: jump = external - internal value; on border exter value =0
       //      mean = (external + internal value)/2, on border just internal value
            ( jump(v)*mean(dn(u)) -  jump(u)*mean(dn(v)) 
          + pena*jump(u)*jump(v) ) / nTonEdge 
)
;
func f=1;
func g=0;
Vh u,v;
Xh uu,vv;
problem A(u,v,solver=UMFPACK) = Ans 
- int2d(Th)(f*v) 
- int1d(Th)(g*dn(v)  + pena*g*v) 
;
problem A1(uu,vv,solver=CG) 
= 
 int2d(Th)(dx(uu)*dx(vv)+dy(uu)*dy(vv)) - int2d(Th)(f*vv) + on(1,2,3,4,uu=g);

 A; // solve  DG
 A1; // solve continuous

plot(u,uu,cmm="Discontinue Galerkin",wait=1,value=1);
plot(u,cmm="Discontinue Galerkin",wait=1,value=1,fill=1);