subroutine tridag(a,b,c,d,n,ierr)
      implicit double precision (a-h,o-z)
      dimension a(1),b(1),c(1),d(1),g(1000)

      ierr = 0
      if(b(1) .eq. 0.d0) then 
        ierr = 1
        return
      end if
      bet = b(1)
      d(1) = d(1)/bet
      do j = 2, n
         g(j) = c(j-1)/bet
         bet = b(j)-a(j)*g(j)
         if(bet .eq. 0.d0) then
            ierr = 1
            return
         end if 
         d(j) = (d(j)-a(j)*d(j-1))/bet
      end do
       
      do j = n-1, 1, -1
         d(j) = d(j)-g(j+1)*d(j+1)
      end do

      return
      end