C P501      SOURCE    CHAT      05/01/13    02:08:37     5004           
      SUBROUTINE P501(D,U,G,R,N,DT,G1)
      IMPLICIT INTEGER(I-N)
      IMPLICIT REAL*8 (A-H,O-Z)
      REAL*8 D(N),U(N),G(N),R(N),G1(N)
      DO 1 I=1,N
      R(I)=(U(I)*D(I)-DT*G(I))/(D(I)+DT*G1(I))
 1    CONTINUE
      RETURN
      END