SUBROUTINE TRIDAG(A,B,C,R,N)
      implicit double precision (a-h,o-z)
      PARAMETER(NMAX=500)
      DIMENSION GAM(NMAX),A(N),B(N),C(N),R(N)
c      IF (B(1).EQ.0.) then
c        write(6,*) 'b(1)=0'
c        PAUSE
c      endif
      BET=B(1)
      R(1)=R(1)/B(1)
      DO 11 J=2,N
        GAM(J)=C(J-1)/BET
        BET=B(J)-A(J)*GAM(J)
c        IF(BET.EQ.0.)PAUSE
        R(J)=(R(J)-A(J)*R(J-1))/BET
11    CONTINUE
      DO 12 J=N-1,1,-1
        R(J)=R(J)-GAM(J+1)*R(J+1)
12    CONTINUE
      RETURN
      END