NAME

       DDTTRF  -  compute an LU factorization of a complex tridiagonal matrix A using elimination
       without partial pivoting

SYNOPSIS

       SUBROUTINE DDTTRF( N, DL, D, DU, INFO )

           INTEGER        INFO, N

           DOUBLE         PRECISION D( * ), DL( * ), DU( * )

PURPOSE

       DDTTRF computes an LU factorization of a complex tridiagonal matrix  A  using  elimination
       without partial pivoting.

       The factorization has the form
          A = L * U
       where L is a product of unit lower bidiagonal
       matrices  and  U  is  upper  triangular  with nonzeros in only the main diagonal and first
       superdiagonal.

ARGUMENTS

       N       (input) INTEGER
               The order of the matrix A.  N >= 0.

       DL      (input/output) COMPLEX array, dimension (N-1)
               On entry, DL must contain the (n-1) subdiagonal elements of A.   On  exit,  DL  is
               overwritten  by  the  (n-1)  multipliers  that  define  the  matrix  L from the LU
               factorization of A.

       D       (input/output) COMPLEX array, dimension (N)
               On entry, D must contain the diagonal elements of A.  On exit, D is overwritten by
               the n diagonal elements of the upper triangular matrix U from the LU factorization
               of A.

       DU      (input/output) COMPLEX array, dimension (N-1)
               On entry, DU must contain the (n-1) superdiagonal elements of A.  On exit,  DU  is
               overwritten by the (n-1) elements of the first superdiagonal of U.

       INFO    (output) INTEGER
               = 0:  successful exit
               < 0:  if INFO = -i, the i-th argument had an illegal value
               >  0:   if INFO = i, U(i,i) is exactly zero. The factorization has been completed,
               but the factor U is exactly singular, and division by zero will  occur  if  it  is
               used to solve a system of equations.