subroutine error(problem,f,mmax,nmax,m,N,a,b,c,d,c0,emax,el2,erel)

      implicit none
      external u

      integer N,problem,mmax,nmax,m
      double precision a,b,c,d,emax,el2,erel,c0,f(mmax+2,nmax+2)

      double precision sum1,e1,sum2,e2,x,y,v,w,u
      integer i,j

      sum1 = 0
      sum2 = 0
      emax = 0
C We do not use the boundary
      do i = 2,m
         do j = 2,n
            x = a + (i-1)*(b-a)/(m+1)
            y = c + (j-1)*(d-c)/(n+1)
            v = a + (i-1+1)*(b-a)/(m+1)
            w = c + (j-1+1)*(d-c)/(n+1)
            if(i .lt. m .and. j .lt. n) then
               e1 = ((u(problem,x,y,c0)-f(i,j)+u(problem,x,w,c0)-f(i+1,j
     $              )+u(problem,v,y,c0)-f(i+1,j+1)+u(problem,v,w,c0)-f(i
     $              ,j+1))/4)**2*(b-a)*(d-c)/((m-1)*(n-1))
               
            
               e2 = ((u(problem,x,y,c0)+u(problem,x,w,c0)+u(problem,v,y,
     $              c0)+u(problem,v,w,c0))/4)**2*(b-a)*(d-c)/((m-1)*(n-1))
               
               sum1 = sum1 + e1
               sum2 = sum2 + e2            
            end if
            if(abs(u(problem,x,y,c0)-f(i,j)) .gt. emax) then
               emax = abs(u(problem,x,y,c0)-f(i,j))
            end if
         end do
      end do
      el2  = sqrt(sum1) 
      erel = sqrt(sum1/sum2)       
      return
      end