# sget52.f - Man Page

TESTING/EIG/sget52.f

## Synopsis

### Functions/Subroutines

subroutine sget52 (left, n, a, lda, b, ldb, e, lde, alphar, alphai, beta, work, result)
SGET52

## Function/Subroutine Documentation

### subroutine sget52 (logical left, integer n, real, dimension( lda, * ) a, integer lda, real, dimension( ldb, * ) b, integer ldb, real, dimension( lde, * ) e, integer lde, real, dimension( * ) alphar, real, dimension( * ) alphai, real, dimension( * ) beta, real, dimension( * ) work, real, dimension( 2 ) result)

SGET52

Purpose:

``` SGET52  does an eigenvector check for the generalized eigenvalue
problem.

The basic test for right eigenvectors is:

| b(j) A E(j) -  a(j) B E(j) |
RESULT(1) = max   -------------------------------
j    n ulp max( |b(j) A|, |a(j) B| )

using the 1-norm.  Here, a(j)/b(j) = w is the j-th generalized
eigenvalue of A - w B, or, equivalently, b(j)/a(j) = m is the j-th
generalized eigenvalue of m A - B.

For real eigenvalues, the test is straightforward.  For complex
eigenvalues, E(j) and a(j) are complex, represented by
Er(j) + i*Ei(j) and ar(j) + i*ai(j), resp., so the test for that
eigenvector becomes

max( |Wr|, |Wi| )
--------------------------------------------
n ulp max( |b(j) A|, (|ar(j)|+|ai(j)|) |B| )

where

Wr = b(j) A Er(j) - ar(j) B Er(j) + ai(j) B Ei(j)

Wi = b(j) A Ei(j) - ai(j) B Er(j) - ar(j) B Ei(j)

T   T  _
For left eigenvectors, A , B , a, and b  are used.

SGET52 also tests the normalization of E.  Each eigenvector is
supposed to be normalized so that the maximum 'absolute value'
of its elements is 1, where in this case, 'absolute value'
of a complex value x is  |Re(x)| + |Im(x)| ; let us call this
maximum 'absolute value' norm of a vector v  M(v).
if a(j)=b(j)=0, then the eigenvector is set to be the jth coordinate
vector.  The normalization test is:

RESULT(2) =      max       | M(v(j)) - 1 | / ( n ulp )
eigenvectors v(j)```
Parameters

LEFT

```          LEFT is LOGICAL
=.TRUE.:  The eigenvectors in the columns of E are assumed
to be *left* eigenvectors.
=.FALSE.: The eigenvectors in the columns of E are assumed
to be *right* eigenvectors.```

N

```          N is INTEGER
The size of the matrices.  If it is zero, SGET52 does
nothing.  It must be at least zero.```

A

```          A is REAL array, dimension (LDA, N)
The matrix A.```

LDA

```          LDA is INTEGER
The leading dimension of A.  It must be at least 1
and at least N.```

B

```          B is REAL array, dimension (LDB, N)
The matrix B.```

LDB

```          LDB is INTEGER
The leading dimension of B.  It must be at least 1
and at least N.```

E

```          E is REAL array, dimension (LDE, N)
The matrix of eigenvectors.  It must be O( 1 ).  Complex
eigenvalues and eigenvectors always come in pairs, the
eigenvalue and its conjugate being stored in adjacent
elements of ALPHAR, ALPHAI, and BETA.  Thus, if a(j)/b(j)
and a(j+1)/b(j+1) are a complex conjugate pair of
generalized eigenvalues, then E(,j) contains the real part
of the eigenvector and E(,j+1) contains the imaginary part.
Note that whether E(,j) is a real eigenvector or part of a
complex one is specified by whether ALPHAI(j) is zero or not.```

LDE

```          LDE is INTEGER
The leading dimension of E.  It must be at least 1 and at
least N.```

ALPHAR

```          ALPHAR is REAL array, dimension (N)
The real parts of the values a(j) as described above, which,
along with b(j), define the generalized eigenvalues.
Complex eigenvalues always come in complex conjugate pairs
a(j)/b(j) and a(j+1)/b(j+1), which are stored in adjacent
elements in ALPHAR, ALPHAI, and BETA.  Thus, if the j-th
and (j+1)-st eigenvalues form a pair, ALPHAR(j+1)/BETA(j+1)
is assumed to be equal to ALPHAR(j)/BETA(j).```

ALPHAI

```          ALPHAI is REAL array, dimension (N)
The imaginary parts of the values a(j) as described above,
which, along with b(j), define the generalized eigenvalues.
If ALPHAI(j)=0, then the eigenvalue is real, otherwise it
is part of a complex conjugate pair.  Complex eigenvalues
always come in complex conjugate pairs a(j)/b(j) and
a(j+1)/b(j+1), which are stored in adjacent elements in
ALPHAR, ALPHAI, and BETA.  Thus, if the j-th and (j+1)-st
eigenvalues form a pair, ALPHAI(j+1)/BETA(j+1) is assumed to
be equal to  -ALPHAI(j)/BETA(j).  Also, nonzero values in
ALPHAI are assumed to always come in adjacent pairs.```

BETA

```          BETA is REAL array, dimension (N)
The values b(j) as described above, which, along with a(j),
define the generalized eigenvalues.```

WORK

`          WORK is REAL array, dimension (N**2+N)`

RESULT

```          RESULT is REAL array, dimension (2)
The values computed by the test described above.  If A E or
B E is likely to overflow, then RESULT(1:2) is set to
10 / ulp.```
Author

Univ. of Tennessee

Univ. of California Berkeley

NAG Ltd.

Definition at line 197 of file sget52.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page sget52(3) is an alias of sget52.f(3).

Tue Nov 28 2023 12:08:42 Version 3.12.0 LAPACK