Purpose ======= LA_PTSVX computes the solution to a linear system of equations A*X = B, where A has tridiagonal form and is real symmetric or complex Hermitian and, in either case, positive definite, and where X and B are rectangular matrices or vectors. LA_PTSVX can also optionally estimate the condition number of A and compute error bounds. ========= SUBROUTINE LA_PTSVX( D, E, B, X, DF=df, EF=ef, FACT=fact, & FERR=ferr, BERR=berr, RCOND=rcond, INFO=info ) REAL(), INTENT(IN) :: D(:) (), INTENT(IN) :: E(:), (), INTENT(OUT) :: REAL(), INTENT(INOUT), OPTIONAL :: DF(:) (), INTENT(INOUT), OPTIONAL :: EF(:) CHARACTER(LEN=1), INTENT(IN), OPTIONAL :: FACT REAL(), INTENT(OUT), OPTIONAL :: , RCOND INTEGER, INTENT(OUT), OPTIONAL :: INFO where ::= REAL | COMPLEX ::= KIND(1.0) | KIND(1.0D0) ::= B(:,:) | B(:) ::= X(:,:) | X(:) ::= FERR(:), BERR(:) | FERR, BERR Arguments ========= D (input) REAL array, shape (:) with size(D) = n, where n is the order of A. The diagonal of A. E (input) REAL or COMPLEX array, shape (:) with size(E) = n-1. The subdiagonal of A. B (input) REAL or COMPLEX array, shape (:,:) with size(B,1) = n or shape (:) with size(B) = n. The matrix B. X (output) REAL or COMPLEX array, shape (:,:) with size(X,1) = n and size(X,2) = size(B,2), or shape (:) with size(X) = n. The solution matrix X . DF Optional (input or output) REAL array, shape (:) with the same size as D. If FACT = 'F', then DF is an input argument that contains the diagonal of D from the L*D*L^H factorization of A. If FACT = 'N', then DF is an output argument that contains the diagonal of D from the L*D*L^H factorization of A. EF Optional (input or output) REAL or COMPLEX array, shape (:) with the same size as E. If FACT = 'F', then EF is an input argument that contains the subdiagonal of L from the L*D*L^H factorization of A. If FACT = 'N', then EF is an output argument that contains the subdiagonal of L from the L*D*L^H factorization of A. FACT Optional (input) CHARACTER(LEN=1). Specifies whether the factored form of A has been supplied on entry. = 'N': The matrix A will be copied to DF and EF and factored. = 'F': DF and EF contain the factored form of A. Default value: 'N'. FERR Optional (output) REAL array of shape (:), with size(FERR) = size(X,2), or REAL scalar. 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). BERR Optional (output) REAL array of shape (:), with size(BERR) = size(X,2), or REAL scalar. 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). RCOND Optional (output) REAL. The estimate of the reciprocal condition number of the matrix A. If RCOND is less than the machine precision, the matrix is singular to working precision. This condition is indicated by a return code of INFO > 0. INFO Optional (output) INTEGER = 0: successful exit. < 0: if INFO = -i, the i-th argument had an illegal value. > 0: if INFO = i, and i is <= n: the leading minor of order i of A is not positive definite, so the factorization could not be completed unless i = n, and the solution and error bounds could not be computed. RCOND = 0 is returned. = n+1: L is nonsingular, but RCOND is less than machine precision, so the matrix is singular to working precision. Nevertheless, the solution and error bounds are computed because the computed solution can be more accurate than the value of RCOND would suggest. If INFO is not present and an error occurs, then the program is terminated with an error message.