---
name: "cungtr.f"
section: "3"
platform: "linux"
package: "lapack-doc"
version: "3.12.1-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/cungtr.f"
---

# cungtr.f(3)

## NAME

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.