Purpose ======= LA_GTSVX computes the solution to a real or complex linear system of equations of the form A*X = B, A^T*X = B or A^H*X = B, where A is a square tridiagonal matrix and X and B are rectangular matrices or vectors. LA_GTSVX can also optionally estimate the condition number of A and compute error bounds. ========= SUBROUTINE LA_GTSVX( DL, D, DU, B, X, DLF=dlf, DF=df, DUF=duf, & DU2=du2, IPIV=ipiv, FACT=fact, TRANS=trans, FERR=ferr, & BERR=berr, RCOND=rcond, INFO=info ) (), INTENT(IN) :: DL(:), D(:), DU(:), (), INTENT(OUT) :: (), INTENT(INOUT), OPTIONAL :: DLF(:), DF(:), & DUF(:), DU2(:) INTEGER, INTENT(INOUT), OPTIONAL :: IPIV(:) CHARACTER(LEN=1), INTENT(IN), OPTIONAL :: FACT, TRANS REAL(), INTENT(OUT), OPTIONAL :: 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 ========= DL (input) REAL or COMPLEX array, shape (:) with size(DL) = n-1. The subdiagonal of A. D (input) REAL or COMPLEX array, shape (:) with size(D) = n. The diagonal of A. DU (input) REAL or COMPLEX array, shape (:) with size(DU) = n-1. The superdiagonal of A. B (input/output) 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 . DLF Optional (input or output) REAL or COMPLEX array, shape (:) with size(DLF)= n-1. If FACT = 'F' then DLF is an input argument that contains the multipliers that define the matrix L from the LU factorization of A. If FACT = 'N' then DLF is an output argument that contains the multipliers that define the matrix L from the LU factorization of A. DF Optional (input or output) REAL or COMPLEX array, shape (:) with size(DF)= n. If FACT = 'F' then DF is an input argument that contains the diagonal of the matrix U . If FACT = 'N' then DF is an output argument that contains the diagonal of the matrix U . DUF Optional (input or output) REAL or COMPLEX array, shape (:) with size(DUF) = n-1. If FACT = 'F' then DUF is an input argument that contains the first superdiagonal of U. If FACT = 'N' then DUF is an output argument that contains the first superdiagonal of U. DU2 Optional (input or output) REAL or COMPLEX array, shape (:) with size(DU2) = n-2. If FACT = 'F', then DU2 is an input argument that contains the second superdiagonal of U. If FACT = 'N', then DU2 is an output argument that contains the second superdiagonal of U. IPIV Optional (input or output) INTEGER array, shape (:) with size(IPIV) = n. If FACT = 'F' then IPIV is an input argument that contains the pivot indices from the LU factorization of A. If FACT = 'N', then IPIV is an output argument that contains the pivot indices from the LU factorization of A; row i of the matrix was interchanged with row IPIV(i). IPIV(i) will always be either i or i+1; IPIV(i) = i indicates a row interchange was not required. FACT Optional (input) CHARACTER(LEN=1). Specifies whether the factored form of A is supplied on entry. = 'N': The matrix will be copied to DLF, DF and DUF and factored. = 'F': DLF, DF, DUF, DU2 and IPIV contain the factored form of A. Default value: 'N'. TRANS Optional (input) CHARACTER(LEN=1). Specifies the form of the system of equations: = 'N': A*X = B (No transpose) = 'T': A^T*X = B (Transpose) = 'C': A^H*X = B (Conjugate transpose) 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). The estimate is as reliable as the estimate for RCOND and is almost always a slight overestimate of the true error. 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: U(i,i) = 0. The factorization has not been completed unless i = n. The factor U is singular, so the solution could not be computed. = n+1: U is nonsingular, but RCOND is less than machine precision, meaning that 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.