?opgtr

Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

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ID 766686

Date 12/16/2022

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?opgtr

Generates the real orthogonal matrix Q determined by ?sptrd.

Syntax

call sopgtr(uplo, n, ap, tau, q, ldq, work, info)

call dopgtr(uplo, n, ap, tau, q, ldq, work, info)

call opgtr(ap, tau, q [,uplo] [,info])

Include Files

mkl.fi, lapack.f90

Description

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

Input Parameters

uplo

CHARACTER*1. Must be 'U' or 'L'. Use the same uplo as supplied to ?sptrd.

n

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

ap, tau

REAL for sopgtr

DOUBLE PRECISION for dopgtr.

Arrays ap and tau, as returned by ?sptrd.

The size of ap must be at least max(1, n(n+1)/2).

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

ldq

INTEGER. The leading dimension of the output array q; at least max(1, n).

work

REAL for sopgtr

DOUBLE PRECISION for dopgtr.

Workspace array, size at least max(1, n-1).

Output Parameters

q

REAL for sopgtr

DOUBLE PRECISION for dopgtr.

Array, size (ldq,*) .

Contains the computed matrix Q.

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

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 opgtr interface are the following:

ap: Holds the array A of size (n*(n+1)/2).
tau: Holds the vector with the number of elements n - 1.
q: Holds the matrix Q of size (n,n).
uplo: Must be 'U' or 'L'. The default value is 'U'.

Application Notes

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 upgtr.

Parent topic: Symmetric Eigenvalue Problems: LAPACK Computational Routines

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Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

?opgtr