---
name: "claqp2"
section: "3lapack"
platform: "linux"
package: "liblapack-doc"
version: "3.2.1-8"
release: "Debian 6.0 (Squeeze)"
platforms: ["linux"]
url: "https://new.carta.tech/claqp2/3lapack"
---

# claqp2(3lapack)

computes a QR factorization with column pivoting of the block A(OFFSET+1:M,1:N)

## NAME

CLAQP2 - computes a QR factorization with column pivoting of the block A(OFFSET+1:M,1:N)

## SYNOPSIS

- SUBROUTINE CLAQP2(

  M, N, OFFSET, A, LDA, JPVT, TAU, VN1, VN2, WORK )

- INTEGER

  LDA, M, N, OFFSET

- INTEGER

  JPVT( \* )

- REAL

  VN1( \* ), VN2( \* )

- COMPLEX

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

## PURPOSE

CLAQP2 computes a QR factorization with column pivoting of the block A(OFFSET+1:M,1:N). The block A(1:OFFSET,1:N) is accordingly pivoted, but not factorized.

## ARGUMENTS

- M (input) INTEGER

  The number of rows of the matrix A. M \>= 0.

- N (input) INTEGER

  The number of columns of the matrix A. N \>= 0.

- OFFSET (input) INTEGER

  The number of rows of the matrix A that must be pivoted but no factorized. OFFSET \>= 0.

- A (input/output) COMPLEX array, dimension (LDA,N)

  On entry, the M-by-N matrix A. On exit, the upper triangle of block A(OFFSET+1:M,1:N) is  the triangular factor obtained; the elements in block A(OFFSET+1:M,1:N) below the diagonal, together with the array TAU, represent the orthogonal matrix Q as a product of elementary reflectors. Block A(1:OFFSET,1:N) has been accordingly pivoted, but no factorized.

- LDA (input) INTEGER

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

- JPVT (input/output) INTEGER array, dimension (N)

  On entry, if JPVT(i) .ne. 0, the i-th column of A is permuted to the front of A\*P (a leading column); if JPVT(i) = 0, the i-th column of A is a free column. On exit, if JPVT(i) = k, then the i-th column of A\*P was the k-th column of A.

- TAU (output) COMPLEX array, dimension (min(M,N))

  The scalar factors of the elementary reflectors.

- VN1 (input/output) REAL array, dimension (N)

  The vector with the partial column norms.

- VN2 (input/output) REAL array, dimension (N)

  The vector with the exact column norms.

- WORK (workspace) COMPLEX array, dimension (N)

  ## FURTHER DETAILS

Based on contributions by\
\
   G. Quintana-Orti, Depto. de Informatica, Universidad Jaime I, Spain\
   X. Sun, Computer Science Dept., Duke University, USA\
 Partial column norm updating strategy modified by\
\
   Z. Drmac and Z. Bujanovic, Dept. of Mathematics,\
\
   University of Zagreb, Croatia.\
\
   June 2006.\
 For more details see LAPACK Working Note 176.\