Visible to Intel only — GUID: GUID-67C23C0F-0CD3-41A4-BE4B-E313A268F2D4
Visible to Intel only — GUID: GUID-67C23C0F-0CD3-41A4-BE4B-E313A268F2D4
?hesvx
Uses the diagonal pivoting factorization to compute the solution to the complex system of linear equations with a Hermitian coefficient matrix A, and provides error bounds on the solution.
Syntax
call chesvx( fact, uplo, n, nrhs, a, lda, af, ldaf, ipiv, b, ldb, x, ldx, rcond, ferr, berr, work, lwork, rwork, info )
call zhesvx( fact, uplo, n, nrhs, a, lda, af, ldaf, ipiv, b, ldb, x, ldx, rcond, ferr, berr, work, lwork, rwork, info )
call hesvx( a, b, x [,uplo] [,af] [,ipiv] [,fact] [,ferr] [,berr] [,rcond] [,info] )
Include Files
- mkl.fi, lapack.f90
Description
The routine uses the diagonal pivoting factorization to compute the solution to a complex system of linear equations A*X = B, where A is an n-by-n Hermitian matrix, the columns of matrix B are individual right-hand sides, and the columns of X are the corresponding solutions.
Error bounds on the solution and a condition estimate are also provided.
The routine ?hesvx performs the following steps:
If fact = 'N', the diagonal pivoting method is used to factor the matrix A. The form of the factorization is A = U*D*UH or A = L*D*LH, where U (or L) is a product of permutation and unit upper (lower) triangular matrices, and D is Hermitian and block diagonal with 1-by-1 and 2-by-2 diagonal blocks.
If some di,i= 0, so that D 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.
The system of equations is solved for X using the factored form of A.
Iterative refinement is applied to improve the computed solution matrix and calculate error bounds and backward error estimates for it.
Input Parameters
fact |
CHARACTER*1. Must be 'F' or 'N'. Specifies whether or not the factored form of the matrix A has been supplied on entry. If fact = 'F': on entry, af and ipiv contain the factored form of A. Arrays a, af, and ipiv are not modified. If fact = 'N', the matrix A is copied to af and factored. |
uplo |
CHARACTER*1. Must be 'U' or 'L'. Indicates whether the upper or lower triangular part of A is stored and how A is factored: If uplo = 'U', the array a stores the upper triangular part of the Hermitian matrix A, and A is factored as U*D*UH. If uplo = 'L', the array a stores the lower triangular part of the Hermitian matrix A; A is factored as L*D*LH. |
n |
INTEGER. The order of matrix A; n≥ 0. |
nrhs |
INTEGER. The number of right-hand sides, the number of columns in B; nrhs≥ 0. |
a, af, b, work |
COMPLEX for chesvx DOUBLE COMPLEX for zhesvx. Arrays: a(size lda by *), af(size ldaf by *), b(size ldb by *), work(*). The array a contains the upper or the lower triangular part of the Hermitian matrix A (see uplo). The second dimension of a must be at least max(1,n). The array af is an input argument if fact = 'F'. It contains he block diagonal matrix D and the multipliers used to obtain the factor U or L from the factorization A = U*D*UH or A = L*D*LH as computed by ?hetrf. The second dimension of af must be at least max(1,n). The array b contains the matrix B whose columns are the right-hand sides for the systems of equations. The second dimension of b must be at least max(1, nrhs). work(*) is a workspace array of dimension at least max(1, lwork). |
lda |
INTEGER. The leading dimension of a; lda≥ max(1, n). |
ldaf |
INTEGER. The leading dimension of af; ldaf≥ max(1, n). |
ldb |
INTEGER. The leading dimension of b; ldb≥ max(1, n). |
ipiv |
INTEGER. Array, size at least max(1, n). The array ipiv is an input argument if fact = 'F'. It contains details of the interchanges and the block structure of D, as determined by ?hetrf. If ipiv(i) = k > 0, then dii is a 1-by-1 diagonal block, and the i-th row and column of A was interchanged with the k-th row and column. If uplo = 'U'and ipiv(i) =ipiv(i-1) = -m < 0, then D has a 2-by-2 block in rows/columns i and i-1, and (i-1)-th row and column of A was interchanged with the m-th row and column. If uplo = 'L'and ipiv(i) =ipiv(i+1) = -m < 0, then D has a 2-by-2 block in rows/columns i and i+1, and (i+1)-th row and column of A was interchanged with the m-th row and column. |
ldx |
INTEGER. The leading dimension of the output array x; ldx≥ max(1, n). |
lwork |
INTEGER. The size of the work array. If lwork = -1, then a workspace query is assumed; the routine only calculates the optimal size of the work array, returns this value as the first entry of the work array, and no error message related to lwork is issued by xerbla. See Application Notes below for details and for the suggested value of lwork. |
rwork |
REAL for chesvx DOUBLE PRECISION for zhesvx. Workspace array, size at least max(1, n). |
Output Parameters
x |
COMPLEX for chesvx DOUBLE COMPLEX for zhesvx. Array, size ldx by *. If info = 0 or info = n+1, the array x contains the solution matrix X to the system of equations. The second dimension of x must be at least max(1,nrhs). |
af, ipiv |
These arrays are output arguments if fact = 'N'. See the description of af, ipiv in Input Arguments section. |
rcond |
REAL for chesvx DOUBLE PRECISION for zhesvx. An 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 |
REAL for chesvx DOUBLE PRECISION for zhesvx. Array, size at least max(1, nrhs). Contains 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 rcon, and is almost always a slight overestimate of the true error. |
berr |
REAL for chesvx DOUBLE PRECISION for zhesvx. Array, size at least max(1, nrhs). Contains the component-wise relative backward error for each solution vector x(j), that is, the smallest relative change in any element of A or B that makes x(j) an exact solution. |
work(1) |
If info = 0, on exit work(1) contains the minimum value of lwork required for optimum performance. Use this lwork for subsequent runs. |
info |
INTEGER. If info = 0, the execution is successful. If info = -i, the i-th parameter had an illegal value. If info = i, and i≤n, then dii is exactly zero. The factorization has been completed, but the block diagonal matrix D is exactly singular, so the solution and error bounds could not be computed; rcond = 0 is returned. If info = i, and i = n + 1, then D 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. |
LAPACK 95 Interface Notes
Routines in Fortran 95 interface have fewer arguments in the calling sequence than their FORTRAN 77 counterparts. For general conventions applied to skip redundant or reconstructible arguments, see LAPACK 95 Interface Conventions.
Specific details for the routine hesvx interface are as follows:
a |
Holds the matrix A of size (n,n). |
b |
Holds the matrix B of size (n,nrhs). |
x |
Holds the matrix X of size (n,nrhs). |
af |
Holds the matrix AF of size (n,n). |
ipiv |
Holds the vector of length n. |
ferr |
Holds the vector of length (nrhs). |
berr |
Holds the vector of length (nrhs). |
uplo |
Must be 'U' or 'L'. The default value is 'U'. |
fact |
Must be 'N' or 'F'. The default value is 'N'. If fact = 'F', then both arguments af and ipiv must be present; otherwise, an error is returned. |
Application Notes
The value of lwork must be at least 2*n. For better performance, try using lwork = n*blocksize, where blocksize is the optimal block size for ?hetrf.
If you are in doubt how much workspace to supply, use a generous value of lwork for the first run or set lwork = -1.
If you choose the first option and set any of admissible lwork sizes, which is no less than the minimal value described, the routine completes the task, though probably not so fast as with a recommended workspace, and provides the recommended workspace in the first element of the corresponding array work on exit. Use this value (work(1)) for subsequent runs.
If you set lwork = -1, the routine returns immediately and provides the recommended workspace in the first element of the corresponding array (work). This operation is called a workspace query.
Note that if you set lwork to less than the minimal required value and not -1, the routine returns immediately with an error exit and does not provide any information on the recommended workspace.