--- name: "doubleGTsolve" section: "3" platform: "linux" package: "liblapack-doc" version: "3.11.0-2" release: "Debian 12 (Bookworm)" platforms: ["linux"] url: "https://new.carta.tech/doubleGTsolve" --- # doubleGTsolve(3) double ## NAME doubleGTsolve - double ## SYNOPSIS ### Functions subroutine **dgtsv** (N, NRHS, DL, D, DU, B, LDB, INFO)\ **DGTSV computes the solution to system of linear equations A \* X = B for GT matrices** subroutine **dgtsvx** (FACT, TRANS, N, NRHS, DL, D, DU, DLF, DF, DUF, DU2, IPIV, B, LDB, X, LDX, RCOND, FERR, BERR, WORK, IWORK, INFO)\ **DGTSVX computes the solution to system of linear equations A \* X = B for GT matrices** ## Detailed Description This is the group of double solve driver functions for GT matrices ## Function Documentation ### subroutine dgtsv (integer N, integer NRHS, double precision, dimension( \* ) DL, double precision, dimension( \* ) D, double precision, dimension( \* ) DU, double precision, dimension( ldb, \* ) B, integer LDB, integer INFO) **DGTSV computes the solution to system of linear equations A \* X = B for GT matrices** **Purpose:** ``` DGTSV solves the equation A*X = B, where A is an n by n tridiagonal matrix, by Gaussian elimination with partial pivoting. Note that the equation A**T*X = B may be solved by interchanging the order of the arguments DU and DL. ``` **Parameters** *N* ``` N is INTEGER The order of the matrix A. N >= 0. ``` *NRHS* ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrix B. NRHS >= 0. ``` *DL* ``` DL is DOUBLE PRECISION array, dimension (N-1) On entry, DL must contain the (n-1) sub-diagonal elements of A. On exit, DL is overwritten by the (n-2) elements of the second super-diagonal of the upper triangular matrix U from the LU factorization of A, in DL(1), ..., DL(n-2). ``` *D* ``` D is DOUBLE PRECISION array, dimension (N) On entry, D must contain the diagonal elements of A. On exit, D is overwritten by the n diagonal elements of U. ``` *DU* ``` DU is DOUBLE PRECISION array, dimension (N-1) On entry, DU must contain the (n-1) super-diagonal elements of A. On exit, DU is overwritten by the (n-1) elements of the first super-diagonal of U. ``` *B* ``` B is DOUBLE PRECISION array, dimension (LDB,NRHS) On entry, the N by NRHS matrix of right hand side matrix B. On exit, if INFO = 0, the N by NRHS solution matrix X. ``` *LDB* ``` LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N). ``` *INFO* ``` INFO is 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, and the solution has not been computed. The factorization has not been completed unless i = N. ``` **Author** Univ. of Tennessee Univ. of California Berkeley Univ. of Colorado Denver NAG Ltd. ### subroutine dgtsvx (character FACT, character TRANS, integer N, integer NRHS, double precision, dimension( \* ) DL, double precision, dimension( \* ) D, double precision, dimension( \* ) DU, double precision, dimension( \* ) DLF, double precision, dimension( \* ) DF, double precision, dimension( \* ) DUF, double precision, dimension( \* ) DU2, integer, dimension( \* ) IPIV, double precision, dimension( ldb, \* ) B, integer LDB, double precision, dimension( ldx, \* ) X, integer LDX, double precision RCOND, double precision, dimension( \* ) FERR, double precision, dimension( \* ) BERR, double precision, dimension( \* ) WORK, integer, dimension( \* ) IWORK, integer INFO) **DGTSVX computes the solution to system of linear equations A \* X = B for GT matrices** **Purpose:** ``` DGTSVX uses the LU factorization to compute the solution to a real system of linear equations A * X = B or A**T * X = B, where A is a tridiagonal matrix of order N and X and B are N-by-NRHS matrices. Error bounds on the solution and a condition estimate are also provided. ``` **Description:** ``` The following steps are performed: 1. If FACT = 'N', the LU decomposition is used to factor the matrix A as A = L * U, where L is a product of permutation and unit lower bidiagonal matrices and U is upper triangular with nonzeros in only the main diagonal and first two superdiagonals. 2. If some U(i,i)=0, so that U is exactly singular, then the routine returns with INFO = i. Otherwise, the factored form of A is used to estimate the condition number of the matrix A. If the reciprocal of the condition number is less than machine precision, INFO = N+1 is returned as a warning, but the routine still goes on to solve for X and compute error bounds as described below. 3. The system of equations is solved for X using the factored form of A. 4. Iterative refinement is applied to improve the computed solution matrix and calculate error bounds and backward error estimates for it. ``` **Parameters** *FACT* ``` FACT is CHARACTER*1 Specifies whether or not the factored form of A has been supplied on entry. = 'F': DLF, DF, DUF, DU2, and IPIV contain the factored form of A; DL, D, DU, DLF, DF, DUF, DU2 and IPIV will not be modified. = 'N': The matrix will be copied to DLF, DF, and DUF and factored. ``` *TRANS* ``` TRANS is CHARACTER*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 = Transpose) ``` *N* ``` N is INTEGER The order of the matrix A. N >= 0. ``` *NRHS* ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrix B. NRHS >= 0. ``` *DL* ``` DL is DOUBLE PRECISION array, dimension (N-1) The (n-1) subdiagonal elements of A. ``` *D* ``` D is DOUBLE PRECISION array, dimension (N) The n diagonal elements of A. ``` *DU* ``` DU is DOUBLE PRECISION array, dimension (N-1) The (n-1) superdiagonal elements of A. ``` *DLF* ``` DLF is DOUBLE PRECISION array, dimension (N-1) If FACT = 'F', then DLF is an input argument and on entry contains the (n-1) multipliers that define the matrix L from the LU factorization of A as computed by DGTTRF. If FACT = 'N', then DLF is an output argument and on exit contains the (n-1) multipliers that define the matrix L from the LU factorization of A. ``` *DF* ``` DF is DOUBLE PRECISION array, dimension (N) If FACT = 'F', then DF is an input argument and on entry contains the n diagonal elements of the upper triangular matrix U from the LU factorization of A. If FACT = 'N', then DF is an output argument and on exit contains the n diagonal elements of the upper triangular matrix U from the LU factorization of A. ``` *DUF* ``` DUF is DOUBLE PRECISION array, dimension (N-1) If FACT = 'F', then DUF is an input argument and on entry contains the (n-1) elements of the first superdiagonal of U. If FACT = 'N', then DUF is an output argument and on exit contains the (n-1) elements of the first superdiagonal of U. ``` *DU2* ``` DU2 is DOUBLE PRECISION array, dimension (N-2) If FACT = 'F', then DU2 is an input argument and on entry contains the (n-2) elements of the second superdiagonal of U. If FACT = 'N', then DU2 is an output argument and on exit contains the (n-2) elements of the second superdiagonal of U. ``` *IPIV* ``` IPIV is INTEGER array, dimension (N) If FACT = 'F', then IPIV is an input argument and on entry contains the pivot indices from the LU factorization of A as computed by DGTTRF. If FACT = 'N', then IPIV is an output argument and on exit 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. ``` *B* ``` B is DOUBLE PRECISION array, dimension (LDB,NRHS) The N-by-NRHS right hand side matrix B. ``` *LDB* ``` LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N). ``` *X* ``` X is DOUBLE PRECISION array, dimension (LDX,NRHS) If INFO = 0 or INFO = N+1, the N-by-NRHS solution matrix X. ``` *LDX* ``` LDX is INTEGER The leading dimension of the array X. LDX >= max(1,N). ``` *RCOND* ``` RCOND is DOUBLE PRECISION The estimate of the reciprocal condition number of the matrix A. If RCOND is less than the machine precision (in particular, if RCOND = 0), the matrix is singular to working precision. This condition is indicated by a return code of INFO > 0. ``` *FERR* ``` FERR is 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* ``` BERR is 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* ``` WORK is DOUBLE PRECISION array, dimension (3*N) ``` *IWORK* ``` IWORK is INTEGER array, dimension (N) ``` *INFO* ``` INFO is 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) is exactly zero. The factorization has not been completed unless i = N, but the factor U is exactly singular, so the solution and error bounds could not be computed. RCOND = 0 is returned. = 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 there are a number of situations where the computed solution can be more accurate than the value of RCOND would suggest. ``` **Author** Univ. of Tennessee Univ. of California Berkeley Univ. of Colorado Denver NAG Ltd. ## Author Generated automatically by Doxygen for LAPACK from the source code.