DOUBLE PRECISION FUNCTION FINDN2S(u,nx,ny,nz)
c***
c     get the L_2 Norm of a scalar field
c***
      INCLUDE 'undefined.h'
      INCLUDE 'mpif.h'
      INTEGER nx, ny, nz, i, j, k, ierr00
      DOUBLE PRECISION u(nx,ny,nz), tnpt
      INTRINSIC DSQRT
c***
      ierr00=MPI_SUCCESS
      CALL CHECK_MPI(IERR00,301)
      FINDN2S=0.d0
      do k=2,nz-1
         do j=2,ny-1
            do i=2,nx-1
               FINDN2S = FINDN2S + u(i,j,k)**2 
            enddo
         enddo
      enddo 
      tnpt = DFLOAT((nx-2)*(ny-2)*(nz-2))
      FINDN2S = FINDN2S/tnpt
      CALL CHECK_MPI(IERR00,302)
c***
      if(havempi.eq.0) then 
         FINDN2S = DSQRT(FINDN2S)
         return
      else if(havempi.eq.1) then
         CALL MPI_ALLREDUCE(FINDN2S,norm_total,1,
     &        MPI_DOUBLE_PRECISION,MPI_SUM,COMM2D,IERR00)
         FINDN2S = dsqrt(norm_total/NPROCS)
         CALL CHECK_MPI(IERR00,303)
         return
      else
         stop 'wrong value of havempi'
      endif
      end