# dtplqt2.f

Section: LAPACK (3)
Updated: Tue Nov 14 2017
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dtplqt2.f

## SYNOPSIS

### Functions/Subroutines

subroutine dtplqt2 (M, N, L, A, LDA, B, LDB, T, LDT, INFO)
DTPLQT2 computes a LQ factorization of a real or complex 'triangular-pentagonal' matrix, which is composed of a triangular block and a pentagonal block, using the compact WY representation for Q.

## Function/Subroutine Documentation

### subroutine dtplqt2 (integer M, integer N, integer L, double precision, dimension( lda, * ) A, integer LDA, double precision, dimension( ldb, * ) B, integer LDB, double precision, dimension( ldt, * ) T, integer LDT, integer INFO)

DTPLQT2 computes a LQ factorization of a real or complex 'triangular-pentagonal' matrix, which is composed of a triangular block and a pentagonal block, using the compact WY representation for Q.

Purpose:

``` DTPLQT2 computes a LQ a factorization of a real "triangular-pentagonal"
matrix C, which is composed of a triangular block A and pentagonal block B,
using the compact WY representation for Q.
```

Parameters:

M

```          M is INTEGER
The total number of rows of the matrix B.
M >= 0.
```

N

```          N is INTEGER
The number of columns of the matrix B, and the order of
the triangular matrix A.
N >= 0.
```

L

```          L is INTEGER
The number of rows of the lower trapezoidal part of B.
MIN(M,N) >= L >= 0.  See Further Details.
```

A

```          A is DOUBLE PRECISION array, dimension (LDA,M)
On entry, the lower triangular M-by-M matrix A.
On exit, the elements on and below the diagonal of the array
contain the lower triangular matrix L.
```

LDA

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

B

```          B is DOUBLE PRECISION array, dimension (LDB,N)
On entry, the pentagonal M-by-N matrix B.  The first N-L columns
are rectangular, and the last L columns are lower trapezoidal.
On exit, B contains the pentagonal matrix V.  See Further Details.
```

LDB

```          LDB is INTEGER
The leading dimension of the array B.  LDB >= max(1,M).
```

T

```          T is DOUBLE PRECISION array, dimension (LDT,M)
The N-by-N upper triangular factor T of the block reflector.
See Further Details.
```

LDT

```          LDT is INTEGER
The leading dimension of the array T.  LDT >= max(1,M)
```

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

NAG Ltd.

Date:

June 2017

Further Details:

```  The input matrix C is a M-by-(M+N) matrix

C = [ A ][ B ]

where A is an lower triangular M-by-M matrix, and B is M-by-N pentagonal
matrix consisting of a M-by-(N-L) rectangular matrix B1 left of a M-by-L
upper trapezoidal matrix B2:

B = [ B1 ][ B2 ]
[ B1 ]  <-     M-by-(N-L) rectangular
[ B2 ]  <-     M-by-L lower trapezoidal.

The lower trapezoidal matrix B2 consists of the first L columns of a
N-by-N lower triangular matrix, where 0 <= L <= MIN(M,N).  If L=0,
B is rectangular M-by-N; if M=L=N, B is lower triangular.

The matrix W stores the elementary reflectors H(i) in the i-th row
above the diagonal (of A) in the M-by-(M+N) input matrix C

C = [ A ][ B ]
[ A ]  <- lower triangular M-by-M
[ B ]  <- M-by-N pentagonal

so that W can be represented as

W = [ I ][ V ]
[ I ]  <- identity, M-by-M
[ V ]  <- M-by-N, same form as B.

Thus, all of information needed for W is contained on exit in B, which
we call V above.  Note that V has the same form as B; that is,

W = [ V1 ][ V2 ]
[ V1 ] <-     M-by-(N-L) rectangular
[ V2 ] <-     M-by-L lower trapezoidal.

The rows of V represent the vectors which define the H(i)'s.
The (M+N)-by-(M+N) block reflector H is then given by

H = I - W**T * T * W

where W^H is the conjugate transpose of W and T is the upper triangular
factor of the block reflector.
```

Definition at line 179 of file dtplqt2.f.

## Author

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