# cungtr.f

Section: LAPACK (3)
Updated: Tue Nov 14 2017
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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.

Date:

December 2016

Definition at line 125 of file cungtr.f.

## Author

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