subroutine error(svar,f,N,a,b,c,d,emax,el2,erel)

      implicit none

      integer N
      double precision a,b,c,d,emax,el2,erel
      double precision, dimension(:,:)  :: svar
      double precision, dimension(:,:)  :: f

      double precision sum1,e1,sum2,e2
      integer i,j

      sum1 = 0
      sum2 = 0
      emax = 0
C We do not use the boundary
      do i = 1,2*N+2
         do j = 1,2*N+2
            if(i .lt. 2*N+2 .and. j .lt. 2*N+2) then
               e1 = ((svar(i,j)-f(i,j)+svar(i+1,j)-f(i+1,j)+svar(i+1,j+1
     $              )-f(i+1,j+1)+svar(i,j+1)-f(i,j+1))/4)**2*(b-a)*(d-c)
     $              /((2*N+2)**2)
            
               e2 = ((svar(i,j)+svar(i+1,j)+svar(i+1,j+1)+svar(i,j+1))/4
     $              )**2*(b-a)*(d-c)/((2*N+2)**2)
               sum1 = sum1 + e1
               sum2 = sum2 + e2            
            end if
            if(abs(svar(i,j)-f(i,j)) .gt. emax) then
               emax = abs(svar(i,j)-f(i,j))
            end if
         end do
      end do
      el2  = sqrt(sum1) 
      erel = sqrt(sum1/sum2)       
      return
      end