SUBROUTINE TRIDAG(A,B,C,R,U,N)
C     ==============================
C
C     Solve tridiagonal system of equations
C      from Numerical Recipes (standard Gaussian elimination)
C
      INCLUDE 'PARAMS.FOR'
      INCLUDE 'WINCOM.FOR'
      DIMENSION A(N),B(N),C(N),R(N),U(N)
      DIMENSION GTRID(MKU)
C
      BTRID=B(1)
      U(1)=R(1)/BTRID
      DO J=2,N
        GTRID(J)=C(J-1)/BTRID
        BTRID=B(J)-A(J)*GTRID(J)
        U(J)=(R(J)-A(J)*U(J-1))/BTRID
      ENDDO
      DO J=N-1,1,-1
        U(J)=U(J)-GTRID(J+1)*U(J+1)
      ENDDO
C
      RETURN
      END