sget52.f - Man Page

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

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

Univ. of Colorado Denver

NAG Ltd.

Definition at line 197 of file sget52.f.