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Unindented Program to be prettified using f90ppr
!--------------------------------------------------------------------------------------------------
subroutine tridiag(a, b, c, r, u, n, code)
! Solves for a vector u of length n the tridiagonal linear set ! from numerical recipes
! m u = r, where a, b and c are the three main diagonals of matrix !
! m(n,n), the other terms are 0. r is the right side vector. !
!--------------------------------------------------------------------------------------------------
integer, parameter :: nmax = 100
integer, intent(in) :: n
double precision, intent(in) :: a(n),b(n),c(n),r(n)
double precision, intent(out) :: u(n)
integer, intent(out) :: code
double precision :: bet,gam(nmax)
integer :: j
if(b(1).eq.0.d0) then
code = 1 ! error unless code=0
return
end if
bet = b(1)
u(1 ) = r(1)/bet
do j = 2, n ! decomposition and forward substitution
gam(j) = c(j-1)/bet
bet = b(j)-a(j)*gam(j)
if(bet .eq. 0.d0) then ! algorithm fails
code = 2
return
end if
u(j) = (r(j)-a(j)*u(j-1))/bet
end do
do j = n-1,1,-1 !back substitution
u(j) = u(j) - gam(j+1) * u(j+1)
end do
code = 0
return
end subroutine tridiag
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