---
name: "dlags2"
section: "3lapack"
platform: "linux"
package: "liblapack-doc"
version: "3.2.1-8"
release: "Debian 6.0 (Squeeze)"
platforms: ["linux"]
url: "https://new.carta.tech/dlags2/3lapack"
---

# dlags2(3lapack)

computes 2-by-2 orthogonal matrices U, V and Q, such that if ( UPPER ) then U'\*A\*Q = U'\*( A1 A2 )\*Q = ( x 0 ) ( 0 A3 ) ( x x ) and V'\*B\*Q = V'\*( B1 B2 )\*Q = ( x 0 ) ( 0 B3 ) ( x x ) or if ( .NOT.UPPER ) then U'\*A\*Q = U'\*( A1 0 )\*Q = ( x x ) ( A2 A3 ) ( 0 x ) and V'\*B\*Q = V'\*( B1 0 )\*Q = ( x x ) ( B2 B3 ) ( 0 x ) The rows of the transformed A and B are parallel, where U = ( CSU SNU ), V = ( CSV SNV ), Q = ( CSQ SNQ ) ( -SNU CSU ) ( -SNV CSV ) ( -SNQ CSQ ) Z' denotes the transpose of Z

## NAME

DLAGS2 - computes 2-by-2 orthogonal matrices U, V and Q, such that if ( UPPER ) then   U'\*A\*Q = U'\*( A1 A2 )\*Q = ( x 0 )  ( 0 A3 ) ( x x ) and  V'\*B\*Q = V'\*( B1 B2 )\*Q = ( x 0 )  ( 0 B3 ) ( x x )  or if ( .NOT.UPPER ) then   U'\*A\*Q = U'\*( A1 0 )\*Q = ( x x )  ( A2 A3 ) ( 0 x ) and  V'\*B\*Q = V'\*( B1 0 )\*Q = ( x x )  ( B2 B3 ) ( 0 x )  The rows of the transformed A and B are parallel, where   U = ( CSU SNU ), V = ( CSV SNV ), Q = ( CSQ SNQ )  ( -SNU CSU ) ( -SNV CSV ) ( -SNQ CSQ )  Z' denotes the transpose of Z

## SYNOPSIS

- SUBROUTINE DLAGS2(

  UPPER, A1, A2, A3, B1, B2, B3, CSU, SNU, CSV, SNV, CSQ, SNQ )

- LOGICAL

  UPPER

- DOUBLE

  PRECISION A1, A2, A3, B1, B2, B3, CSQ, CSU, CSV, SNQ, SNU, SNV

## PURPOSE

DLAGS2 computes 2-by-2 orthogonal matrices U, V and Q, such that if ( UPPER ) then

## ARGUMENTS

- UPPER (input) LOGICAL

  \= .TRUE.: the input matrices A and B are upper triangular.\
   \= .FALSE.: the input matrices A and B are lower triangular.

- A1 (input) DOUBLE PRECISION

  A2      (input) DOUBLE PRECISION A3      (input) DOUBLE PRECISION On entry, A1, A2 and A3 are elements of the input 2-by-2 upper (lower) triangular matrix A.

- B1 (input) DOUBLE PRECISION

  B2      (input) DOUBLE PRECISION B3      (input) DOUBLE PRECISION On entry, B1, B2 and B3 are elements of the input 2-by-2 upper (lower) triangular matrix B.

- CSU (output) DOUBLE PRECISION

  SNU     (output) DOUBLE PRECISION The desired orthogonal matrix U.

- CSV (output) DOUBLE PRECISION

  SNV     (output) DOUBLE PRECISION The desired orthogonal matrix V.

- CSQ (output) DOUBLE PRECISION

  SNQ     (output) DOUBLE PRECISION The desired orthogonal matrix Q.