SUBROUTINE TRIDAG(A,B,C,R,U,N)
C     ==============================
C    
C     solution of the tridiagonal system
c
c     from Numerical recipes, Sect 2.4
c
      INCLUDE 'IMPLIC.FOR'
      DIMENSION A(N),B(N),C(N),R(N),U(N),GAM(N)
C
      BET=B(1)
      U(1)=R(1)/BET
      DO J=2,N
         GAM(J)=C(J-1)/BET
         BET=B(J)-A(J)*GAM(J)
         U(J)=(R(J)-A(J)*U(J-1))/BET
      END DO
      DO J=N-1,1,-1
         U(J)=U(J)-GAM(J+1)*U(J+1)
      END DO
      RETURN
      END