# cunghr.f

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

## SYNOPSIS

### Functions/Subroutines

subroutine cunghr (N, ILO, IHI, A, LDA, TAU, WORK, LWORK, INFO)
CUNGHR

## Function/Subroutine Documentation

### subroutine cunghr (integer N, integer ILO, integer IHI, complex, dimension( lda, * ) A, integer LDA, complex, dimension( * ) TAU, complex, dimension( * ) WORK, integer LWORK, integer INFO)

CUNGHR

Purpose:

``` CUNGHR generates a complex unitary matrix Q which is defined as the
product of IHI-ILO elementary reflectors of order N, as returned by
CGEHRD:

Q = H(ilo) H(ilo+1) . . . H(ihi-1).
```

Parameters:

N

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

ILO

```          ILO is INTEGER
```

IHI

```          IHI is INTEGER

ILO and IHI must have the same values as in the previous call
of CGEHRD. Q is equal to the unit matrix except in the
submatrix Q(ilo+1:ihi,ilo+1:ihi).
1 <= ILO <= IHI <= N, if N > 0; ILO=1 and IHI=0, if N=0.
```

A

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

LDA

```          LDA is INTEGER
The leading dimension of the array A. LDA >= max(1,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 CGEHRD.
```

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 >= IHI-ILO.
For optimum performance LWORK >= (IHI-ILO)*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 128 of file cunghr.f.

## Author

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