# claesy.f

Section: LAPACK (3)
Updated: Tue Nov 14 2017
Page Index

claesy.f

## SYNOPSIS

### Functions/Subroutines

subroutine claesy (A, B, C, RT1, RT2, EVSCAL, CS1, SN1)
CLAESY computes the eigenvalues and eigenvectors of a 2-by-2 complex symmetric matrix.

## Function/Subroutine Documentation

### subroutine claesy (complex A, complex B, complex C, complex RT1, complex RT2, complex EVSCAL, complex CS1, complex SN1)

CLAESY computes the eigenvalues and eigenvectors of a 2-by-2 complex symmetric matrix.

Purpose:

``` 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.

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 ]
```

Parameters:

A

```          A is COMPLEX
The ( 1, 1 ) element of input matrix.
```

B

```          B is 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

```          C is COMPLEX
The ( 2, 2 ) element of input matrix.
```

RT1

```          RT1 is COMPLEX
The eigenvalue of larger modulus.
```

RT2

```          RT2 is COMPLEX
The eigenvalue of smaller modulus.
```

EVSCAL

```          EVSCAL is 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 eigenvectors before scaling was larger than the threshold
value THRESH (set below).
```

CS1

```          CS1 is COMPLEX
```

SN1

```          SN1 is COMPLEX
If EVSCAL .NE. 0,  ( CS1, SN1 ) is the unit right eigenvector
for RT1.
```

Author:

Univ. of Tennessee

Univ. of California Berkeley