# cungtr (l) - Linux Manuals

## NAME

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

## SYNOPSIS

SUBROUTINE CUNGTR(
UPLO, N, A, LDA, TAU, WORK, LWORK, INFO )

CHARACTER UPLO

INTEGER INFO, LDA, LWORK, N

COMPLEX A( LDA, * ), TAU( * ), WORK( * )

## 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 = aqUaq, Q = H(n-1) . . . H(2) H(1),
if UPLO = aqLaq, Q = H(1) H(2) . . . H(n-1).

## ARGUMENTS

UPLO (input) CHARACTER*1
= aqUaq: Upper triangle of A contains elementary reflectors from CHETRD; = aqLaq: Lower triangle of A contains elementary reflectors from CHETRD.
N (input) INTEGER
The order of the matrix Q. N >= 0.
A (input/output) 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 (input) INTEGER
The leading dimension of the array A. LDA >= N.
TAU (input) COMPLEX array, dimension (N-1)
TAU(i) must contain the scalar factor of the elementary reflector H(i), as returned by CHETRD.
WORK (workspace/output) COMPLEX array, dimension (MAX(1,LWORK))
On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
LWORK (input) 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 (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value