DPTRFS(l)								 )								 DPTRFS(l)

NAME

       DPTRFS  -  improve  the	computed  solution  to a system of linear equations when the coefficient matrix is symmetric positive definite and
       tridiagonal, and provides error bounds and backward error estimates for the solution

SYNOPSIS

       SUBROUTINE DPTRFS( N, NRHS, D, E, DF, EF, B, LDB, X, LDX, FERR, BERR, WORK, INFO )

	   INTEGER	  INFO, LDB, LDX, N, NRHS

	   DOUBLE	  PRECISION B( LDB, * ), BERR( * ), D( * ), DF( * ), E( * ), EF( * ), FERR( * ), WORK( * ), X( LDX, * )

PURPOSE

       DPTRFS improves the computed solution to a system of linear equations when the coefficient matrix is symmetric positive definite and tridi-
       agonal, and provides error bounds and backward error estimates for the solution.

ARGUMENTS

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

       NRHS    (input) INTEGER
	       The number of right hand sides, i.e., the number of columns of the matrix B.  NRHS >= 0.

       D       (input) DOUBLE PRECISION array, dimension (N)
	       The n diagonal elements of the tridiagonal matrix A.

       E       (input) DOUBLE PRECISION array, dimension (N-1)
	       The (n-1) subdiagonal elements of the tridiagonal matrix A.

       DF      (input) DOUBLE PRECISION array, dimension (N)
	       The n diagonal elements of the diagonal matrix D from the factorization computed by DPTTRF.

       EF      (input) DOUBLE PRECISION array, dimension (N-1)
	       The (n-1) subdiagonal elements of the unit bidiagonal factor L from the factorization computed by DPTTRF.

       B       (input) DOUBLE PRECISION array, dimension (LDB,NRHS)
	       The right hand side matrix B.

       LDB     (input) INTEGER
	       The leading dimension of the array B.  LDB >= max(1,N).

       X       (input/output) DOUBLE PRECISION array, dimension (LDX,NRHS)
	       On entry, the solution matrix X, as computed by DPTTRS.	On exit, the improved solution matrix X.

       LDX     (input) INTEGER
	       The leading dimension of the array X.  LDX >= max(1,N).

       FERR    (output) DOUBLE PRECISION array, dimension (NRHS)
	       The  forward  error  bound for each solution vector X(j) (the j-th column of the solution matrix X).  If XTRUE is the true solution
	       corresponding to X(j), FERR(j) is an estimated upper bound for the magnitude of the largest element in (X(j) -  XTRUE)  divided	by
	       the magnitude of the largest element in X(j).

       BERR    (output) DOUBLE PRECISION array, dimension (NRHS)
	       The componentwise relative backward error of each solution vector X(j) (i.e., the smallest relative change in any element of A or B
	       that makes X(j) an exact solution).

       WORK    (workspace) DOUBLE PRECISION array, dimension (2*N)

       INFO    (output) INTEGER
	       = 0:  successful exit
	       < 0:  if INFO = -i, the i-th argument had an illegal value

PARAMETERS

       ITMAX is the maximum number of steps of iterative refinement.

LAPACK version 3.0						   15 June 2000 							 DPTRFS(l)