SUBROUTINE  initialisation(  u,  f,  u_exact  )
      USE numerics
      IMPLICIT  NONE
      REAL(rp),  DIMENSION(sx-1:ex+1,sy-1:ey+1,sz-1:ez+1,0:1),  INTENT(OUT)  ::  u
      REAL(rp),  DIMENSION(sx-1:ex+1,sy-1:ey+1,sz-1:ez+1),      INTENT(OUT)  ::  f
      REAL(rp),  DIMENSION(sx-1:ex+1,sy-1:ey+1,sz-1:ez+1),      INTENT(OUT)  ::  u_exact
      END  SUBROUTINE  initialisation