?orgtr

Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

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ID 766686

Date 3/31/2023

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?orgtr

Generates the real orthogonal matrix Q determined by ?sytrd.

Syntax

call sorgtr(uplo, n, a, lda, tau, work, lwork, info)

call dorgtr(uplo, n, a, lda, tau, work, lwork, info)

call orgtr(a, tau [,uplo] [,info])

Include Files

mkl.fi, lapack.f90

Description

The routine explicitly generates the n-by-n orthogonal matrix Q formed by ?sytrd when reducing a real symmetric matrix A to tridiagonal form: A = Q*T*Q^T. Use this routine after a call to ?sytrd.

Input Parameters

uplo

CHARACTER*1. Must be 'U' or 'L'.

Use the same uplo as supplied to ?sytrd.

n

INTEGER. The order of the matrix Q (n≥ 0).

a, tau, work

REAL for sorgtr

DOUBLE PRECISION for dorgtr.

Arrays:

a(lda,*) is the array a as returned by ?sytrd.

The second dimension of a must be at least max(1, n).

tau(*) is the array tau as returned by ?sytrd.

The size of tau must be at least max(1, n-1).

work is a workspace array, its dimension max(1, lwork).

lda

INTEGER. The leading dimension of a; at least max(1, n).

lwork

INTEGER. The size of the work array (lwork≥n).

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 for the suggested value of lwork.

Output Parameters

a

Overwritten by the orthogonal matrix Q.

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.

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 restorable arguments, see LAPACK 95 Interface Conventions.

Specific details for the routine orgtr interface are the following:

a: Holds the matrix A of size (n,n).
tau: Holds the vector of length (n-1).
uplo: Must be 'U' or 'L'. The default value is 'U'.

Application Notes

For better performance, try using lwork = (n-1)*blocksize, where blocksize is a machine-dependent value (typically, 16 to 64) required for optimum performance of the blocked algorithm.

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.

The computed matrix Q differs from an exactly orthogonal matrix by a matrix E such that ||E||₂ = O(ε), where ε is the machine precision.

The approximate number of floating-point operations is (4/3)n³.

The complex counterpart of this routine is ungtr.

Parent topic: Symmetric Eigenvalue Problems: LAPACK Computational Routines

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Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

?orgtr