SUBROUTINE  erreur(  u,  u_exact,  gerr,  comm3d  )
      USE numerics
      USE MPI
      IMPLICIT  NONE
      REAL(rp),  INTENT(OUT)   ::  gerr
      REAL(rp),  DIMENSION(  sx-1:ex+1,  sy-1:ey+1,  sz-1:ez+1,  0:1),  INTENT(IN)  ::  u
      REAL(rp),  DIMENSION(  sx-1:ex+1,  sy-1:ey+1,  sz-1:ez+1      ),  INTENT(IN)  ::  u_exact
      INTEGER,  INTENT(IN)  ::  comm3d
      END  SUBROUTINE  erreur