CLAESY(1) LAPACK auxiliary routine (version 3.2) CLAESY(1)NAME
CLAESY - computes the eigendecomposition of a 2-by-2 symmetric matrix
( ( A, B );( B, C ) ) provided the norm of the matrix of eigenvectors
is larger than some threshold value
SYNOPSIS
SUBROUTINE CLAESY( A, B, C, RT1, RT2, EVSCAL, CS1, SN1 )
COMPLEX A, B, C, CS1, EVSCAL, RT1, RT2, SN1
PURPOSE
CLAESY computes the eigendecomposition of a 2-by-2 symmetric matrix
( ( A, B );( B, C ) ) provided the norm of the matrix of eigenvec‐
tors is larger than some threshold value. RT1 is the eigenvalue of
larger absolute value, and RT2 of smaller absolute value. If the
eigenvectors are computed, then on return ( CS1, SN1 ) is the unit
eigenvector for RT1, hence [ CS1 SN1 ] . [ A B ] . [ CS1
-SN1 ] = [ RT1 0 ] [ -SN1 CS1 ] [ B C ] [ SN1 CS1
] [ 0 RT2 ]
ARGUMENTS
A (input) COMPLEX
The ( 1, 1 ) element of input matrix.
B (input) COMPLEX
The ( 1, 2 ) element of input matrix. The ( 2, 1 ) element is
also given by B, since the 2-by-2 matrix is symmetric.
C (input) COMPLEX
The ( 2, 2 ) element of input matrix.
RT1 (output) COMPLEX
The eigenvalue of larger modulus.
RT2 (output) COMPLEX
The eigenvalue of smaller modulus.
EVSCAL (output) COMPLEX
The complex value by which the eigenvector matrix was scaled to
make it orthonormal. If EVSCAL is zero, the eigenvectors were
not computed. This means one of two things: the 2-by-2 matrix
could not be diagonalized, or the norm of the matrix of eigen‐
vectors before scaling was larger than the threshold value
THRESH (set below).
CS1 (output) COMPLEX
SN1 (output) COMPLEX If EVSCAL .NE. 0, ( CS1, SN1 ) is the
unit right eigenvector for RT1.
LAPACK auxiliary routine (versioNovember 2008 CLAESY(1)
