subroutine exchng1( a, nx, s, e, comm1d, nbrbottom, nbrtop ) 
	include 'mpif.h' 
	integer nx, s, e 
	double precision a(0:nx+1,s-1:e+1) 
	integer comm1d, nbrbottom, nbrtop 
	integer status(MPI_STATUS_SIZE), ierr 
C 
	call MPI_SEND( a(1,e), nx, MPI_DOUBLE_PRECISION,  
     *                  nbrtop, 0, comm1d, ierr ) 
	call MPI_RECV( a(1,s-1), nx, MPI_DOUBLE_PRECISION,  
     *                 nbrbottom, 0, comm1d, status, ierr ) 
	call MPI_SEND( a(1,s), nx, MPI_DOUBLE_PRECISION,  
     *                 nbrbottom, 1, comm1d, ierr ) 
	call MPI_RECV( a(1,e+1), nx, MPI_DOUBLE_PRECISION,  
     *                 nbrtop, 1, comm1d, status, ierr ) 
	return 
	end