# cungtr.f - Man Page

SRC/cungtr.f

## Synopsis

### Functions/Subroutines

subroutine **cungtr** (uplo, n, a, lda, tau, work, lwork, info)**CUNGTR**

## Function/Subroutine Documentation

### subroutine cungtr (character uplo, integer n, complex, dimension( lda, * ) a, integer lda, complex, dimension( * ) tau, complex, dimension( * ) work, integer lwork, integer info)

**CUNGTR**

**Purpose:**

CUNGTR generates a complex unitary matrix Q which is defined as the product of n-1 elementary reflectors of order N, as returned by CHETRD: if UPLO = 'U', Q = H(n-1) . . . H(2) H(1), if UPLO = 'L', Q = H(1) H(2) . . . H(n-1).

**Parameters***UPLO*UPLO is CHARACTER*1 = 'U': Upper triangle of A contains elementary reflectors from CHETRD; = 'L': Lower triangle of A contains elementary reflectors from CHETRD.

*N*N is INTEGER The order of the matrix Q. N >= 0.

*A*A is COMPLEX array, dimension (LDA,N) On entry, the vectors which define the elementary reflectors, as returned by CHETRD. On exit, the N-by-N unitary matrix Q.

*LDA*LDA is INTEGER The leading dimension of the array A. LDA >= N.

*TAU*TAU is COMPLEX array, dimension (N-1) TAU(i) must contain the scalar factor of the elementary reflector H(i), as returned by CHETRD.

*WORK*WORK is COMPLEX array, dimension (MAX(1,LWORK)) On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

*LWORK*LWORK is INTEGER The dimension of the array WORK. LWORK >= N-1. For optimum performance LWORK >= (N-1)*NB, where NB is the optimal blocksize. 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.

*INFO*INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value

**Author**Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line **122** of file **cungtr.f**.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page cungtr(3) is an alias of cungtr.f(3).

Tue Nov 28 2023 12:08:41 Version 3.12.0 LAPACK