CXML
        	    
LAPACK version 3.0
dpprfs(3)

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

SYNTAX
  SUBROUTINE DPPRFS( UPLO, N, NRHS, AP, AFP, B, LDB, X, LDX, FERR, BERR,
		     WORK, IWORK, INFO )

      CHARACTER	     UPLO

      INTEGER	     INFO, LDB, LDX, N, NRHS

      INTEGER	     IWORK( * )

      DOUBLE	     PRECISION AFP( * ), AP( * ), B( LDB, * ), BERR( * ),
		     FERR( * ), WORK( * ), X( LDX, * )

DESCRIPTION
  DPPRFS improves the computed solution to a system of linear equations when
  the coefficient matrix is symmetric positive definite and packed, and
  provides error bounds and backward error estimates for the solution.

ARGUMENTS
  UPLO	  (input) CHARACTER*1
	  = 'U':  Upper triangle of A is stored;
	  = 'L':  Lower triangle of A is stored.

  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
	  matrices B and X.  NRHS >= 0.

  AP	  (input) DOUBLE PRECISION array, dimension (N*(N+1)/2)
	  The upper or lower triangle of the symmetric matrix A, packed
	  columnwise in a linear array.	 The j-th column of A is stored in
	  the array AP as follows: if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j)
	  for 1<=i<=j; if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = A(i,j) for
	  j<=i<=n.

  AFP	  (input) DOUBLE PRECISION array, dimension (N*(N+1)/2)
	  The triangular factor U or L from the Cholesky factorization A =
	  U**T*U or A = L*L**T, as computed by DPPTRF/ZPPTRF, packed
	  columnwise in a linear array in the same format as A (see AP).

  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 DPPTRS.  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 estimated 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).  The estimate is
	  as reliable as the estimate for RCOND, and is almost always a
	  slight overestimate of the true error.

  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 (3*N)

  IWORK	  (workspace) INTEGER array, dimension (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.