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!Environment Canada - Atmospheric Science and Technology License/Disclaimer,
!                     version 3; Last Modified: May 7, 2008.
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!
C
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C

      subroutine tridiag(u,r,a,b,c,n) 2,2
*
*Author: Luc Fillion - MSC- 9 march 04
*                    - Modified form of Numerical Recipies's original code.           
*
      IMPLICIT NONE
      integer n
      real*8 a(n),b(n),c(n),r(n),u(n)
      integer j
      real*8 bet(n),gam(n),zero
!
#include "taglam4d.cdk"
#include "comlun.cdk"
!
!!
      zero = 0.0
      if(b(1).eq.zero) then
         call abort3d(nulout,'TRIDIAG: rewrite your equations!')
      endif
      bet(1)=b(1)
      do j=2,n
        gam(j)=c(j-1)/bet(j-1)
        bet(j)=b(j)-a(j)*gam(j)
        if(bet(j).eq.zero) then
          call abort3d(nulout,'TRIDIAG: tridiag failed')
        endif
      enddo
!
      u(1)=r(1)/bet(1)
      do j=2,n
        u(j)=(r(j)-a(j)*u(j-1))/bet(j)
      enddo
!
      do j=n-1,1,-1
        u(j)=u(j)-gam(j+1)*u(j+1)
      enddo
!
      return
      end