SGGBAK(1) LAPACK routine (version 3.2) SGGBAK(1)NAME
SGGBAK - forms the right or left eigenvectors of a real generalized ei‐
genvalue problem A*x = lambda*B*x, by backward transformation on the
computed eigenvectors of the balanced pair of matrices output by SGGBAL
SYNOPSIS
SUBROUTINE SGGBAK( JOB, SIDE, N, ILO, IHI, LSCALE, RSCALE, M, V, LDV,
INFO )
CHARACTER JOB, SIDE
INTEGER IHI, ILO, INFO, LDV, M, N
REAL LSCALE( * ), RSCALE( * ), V( LDV, * )
PURPOSE
SGGBAK forms the right or left eigenvectors of a real generalized ei‐
genvalue problem A*x = lambda*B*x, by backward transformation on the
computed eigenvectors of the balanced pair of matrices output by SGG‐
BAL.
ARGUMENTS
JOB (input) CHARACTER*1
Specifies the type of backward transformation required:
= 'N': do nothing, return immediately;
= 'P': do backward transformation for permutation only;
= 'S': do backward transformation for scaling only;
= 'B': do backward transformations for both permutation and
scaling. JOB must be the same as the argument JOB supplied to
SGGBAL.
SIDE (input) CHARACTER*1
= 'R': V contains right eigenvectors;
= 'L': V contains left eigenvectors.
N (input) INTEGER
The number of rows of the matrix V. N >= 0.
ILO (input) INTEGER
IHI (input) INTEGER The integers ILO and IHI determined by
SGGBAL. 1 <= ILO <= IHI <= N, if N > 0; ILO=1 and IHI=0, if
N=0.
LSCALE (input) REAL array, dimension (N)
Details of the permutations and/or scaling factors applied to
the left side of A and B, as returned by SGGBAL.
RSCALE (input) REAL array, dimension (N)
Details of the permutations and/or scaling factors applied to
the right side of A and B, as returned by SGGBAL.
M (input) INTEGER
The number of columns of the matrix V. M >= 0.
V (input/output) REAL array, dimension (LDV,M)
On entry, the matrix of right or left eigenvectors to be trans‐
formed, as returned by STGEVC. On exit, V is overwritten by
the transformed eigenvectors.
LDV (input) INTEGER
The leading dimension of the matrix V. LDV >= max(1,N).
INFO (output) INTEGER
= 0: successful exit.
< 0: if INFO = -i, the i-th argument had an illegal value.
FURTHER DETAILS
See R.C. Ward, Balancing the generalized eigenvalue problem,
SIAM J. Sci. Stat. Comp. 2 (1981), 141-152.
LAPACK routine (version 3.2) November 2008 SGGBAK(1)
