.TH "SRC/strevc.f" 3 "Version 3.12.0" "LAPACK" \" -*- nroff -*-
.ad l
.nh
.SH NAME
SRC/strevc.f
.SH SYNOPSIS
.br
.PP
.SS "Functions/Subroutines"

.in +1c
.ti -1c
.RI "subroutine \fBstrevc\fP (side, howmny, select, n, t, ldt, vl, ldvl, vr, ldvr, mm, m, work, info)"
.br
.RI "\fBSTREVC\fP "
.in -1c
.SH "Function/Subroutine Documentation"
.PP 
.SS "subroutine strevc (character side, character howmny, logical, dimension( * ) select, integer n, real, dimension( ldt, * ) t, integer ldt, real, dimension( ldvl, * ) vl, integer ldvl, real, dimension( ldvr, * ) vr, integer ldvr, integer mm, integer m, real, dimension( * ) work, integer info)"

.PP
\fBSTREVC\fP  
.PP
\fBPurpose:\fP
.RS 4

.PP
.nf
!>
!> STREVC computes some or all of the right and/or left eigenvectors of
!> a real upper quasi-triangular matrix T\&.
!> Matrices of this type are produced by the Schur factorization of
!> a real general matrix:  A = Q*T*Q**T, as computed by SHSEQR\&.
!>
!> The right eigenvector x and the left eigenvector y of T corresponding
!> to an eigenvalue w are defined by:
!>
!>    T*x = w*x,     (y**H)*T = w*(y**H)
!>
!> where y**H denotes the conjugate transpose of y\&.
!> The eigenvalues are not input to this routine, but are read directly
!> from the diagonal blocks of T\&.
!>
!> This routine returns the matrices X and/or Y of right and left
!> eigenvectors of T, or the products Q*X and/or Q*Y, where Q is an
!> input matrix\&.  If Q is the orthogonal factor that reduces a matrix
!> A to Schur form T, then Q*X and Q*Y are the matrices of right and
!> left eigenvectors of A\&.
!> 
.fi
.PP
 
.RE
.PP
\fBParameters\fP
.RS 4
\fISIDE\fP 
.PP
.nf
!>          SIDE is CHARACTER*1
!>          = 'R':  compute right eigenvectors only;
!>          = 'L':  compute left eigenvectors only;
!>          = 'B':  compute both right and left eigenvectors\&.
!> 
.fi
.PP
.br
\fIHOWMNY\fP 
.PP
.nf
!>          HOWMNY is CHARACTER*1
!>          = 'A':  compute all right and/or left eigenvectors;
!>          = 'B':  compute all right and/or left eigenvectors,
!>                  backtransformed by the matrices in VR and/or VL;
!>          = 'S':  compute selected right and/or left eigenvectors,
!>                  as indicated by the logical array SELECT\&.
!> 
.fi
.PP
.br
\fISELECT\fP 
.PP
.nf
!>          SELECT is LOGICAL array, dimension (N)
!>          If HOWMNY = 'S', SELECT specifies the eigenvectors to be
!>          computed\&.
!>          If w(j) is a real eigenvalue, the corresponding real
!>          eigenvector is computed if SELECT(j) is \&.TRUE\&.\&.
!>          If w(j) and w(j+1) are the real and imaginary parts of a
!>          complex eigenvalue, the corresponding complex eigenvector is
!>          computed if either SELECT(j) or SELECT(j+1) is \&.TRUE\&., and
!>          on exit SELECT(j) is set to \&.TRUE\&. and SELECT(j+1) is set to
!>          \&.FALSE\&.\&.
!>          Not referenced if HOWMNY = 'A' or 'B'\&.
!> 
.fi
.PP
.br
\fIN\fP 
.PP
.nf
!>          N is INTEGER
!>          The order of the matrix T\&. N >= 0\&.
!> 
.fi
.PP
.br
\fIT\fP 
.PP
.nf
!>          T is REAL array, dimension (LDT,N)
!>          The upper quasi-triangular matrix T in Schur canonical form\&.
!> 
.fi
.PP
.br
\fILDT\fP 
.PP
.nf
!>          LDT is INTEGER
!>          The leading dimension of the array T\&. LDT >= max(1,N)\&.
!> 
.fi
.PP
.br
\fIVL\fP 
.PP
.nf
!>          VL is REAL array, dimension (LDVL,MM)
!>          On entry, if SIDE = 'L' or 'B' and HOWMNY = 'B', VL must
!>          contain an N-by-N matrix Q (usually the orthogonal matrix Q
!>          of Schur vectors returned by SHSEQR)\&.
!>          On exit, if SIDE = 'L' or 'B', VL contains:
!>          if HOWMNY = 'A', the matrix Y of left eigenvectors of T;
!>          if HOWMNY = 'B', the matrix Q*Y;
!>          if HOWMNY = 'S', the left eigenvectors of T specified by
!>                           SELECT, stored consecutively in the columns
!>                           of VL, in the same order as their
!>                           eigenvalues\&.
!>          A complex eigenvector corresponding to a complex eigenvalue
!>          is stored in two consecutive columns, the first holding the
!>          real part, and the second the imaginary part\&.
!>          Not referenced if SIDE = 'R'\&.
!> 
.fi
.PP
.br
\fILDVL\fP 
.PP
.nf
!>          LDVL is INTEGER
!>          The leading dimension of the array VL\&.  LDVL >= 1, and if
!>          SIDE = 'L' or 'B', LDVL >= N\&.
!> 
.fi
.PP
.br
\fIVR\fP 
.PP
.nf
!>          VR is REAL array, dimension (LDVR,MM)
!>          On entry, if SIDE = 'R' or 'B' and HOWMNY = 'B', VR must
!>          contain an N-by-N matrix Q (usually the orthogonal matrix Q
!>          of Schur vectors returned by SHSEQR)\&.
!>          On exit, if SIDE = 'R' or 'B', VR contains:
!>          if HOWMNY = 'A', the matrix X of right eigenvectors of T;
!>          if HOWMNY = 'B', the matrix Q*X;
!>          if HOWMNY = 'S', the right eigenvectors of T specified by
!>                           SELECT, stored consecutively in the columns
!>                           of VR, in the same order as their
!>                           eigenvalues\&.
!>          A complex eigenvector corresponding to a complex eigenvalue
!>          is stored in two consecutive columns, the first holding the
!>          real part and the second the imaginary part\&.
!>          Not referenced if SIDE = 'L'\&.
!> 
.fi
.PP
.br
\fILDVR\fP 
.PP
.nf
!>          LDVR is INTEGER
!>          The leading dimension of the array VR\&.  LDVR >= 1, and if
!>          SIDE = 'R' or 'B', LDVR >= N\&.
!> 
.fi
.PP
.br
\fIMM\fP 
.PP
.nf
!>          MM is INTEGER
!>          The number of columns in the arrays VL and/or VR\&. MM >= M\&.
!> 
.fi
.PP
.br
\fIM\fP 
.PP
.nf
!>          M is INTEGER
!>          The number of columns in the arrays VL and/or VR actually
!>          used to store the eigenvectors\&.
!>          If HOWMNY = 'A' or 'B', M is set to N\&.
!>          Each selected real eigenvector occupies one column and each
!>          selected complex eigenvector occupies two columns\&.
!> 
.fi
.PP
.br
\fIWORK\fP 
.PP
.nf
!>          WORK is REAL array, dimension (3*N)
!> 
.fi
.PP
.br
\fIINFO\fP 
.PP
.nf
!>          INFO is INTEGER
!>          = 0:  successful exit
!>          < 0:  if INFO = -i, the i-th argument had an illegal value
!> 
.fi
.PP
 
.RE
.PP
\fBAuthor\fP
.RS 4
Univ\&. of Tennessee 

.PP
Univ\&. of California Berkeley 

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Univ\&. of Colorado Denver 

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NAG Ltd\&. 
.RE
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\fBFurther Details:\fP
.RS 4

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.nf
!>
!>  The algorithm used in this program is basically backward (forward)
!>  substitution, with scaling to make the the code robust against
!>  possible overflow\&.
!>
!>  Each eigenvector is normalized so that the element of largest
!>  magnitude has magnitude 1; here the magnitude of a complex number
!>  (x,y) is taken to be |x| + |y|\&.
!> 
.fi
.PP
 
.RE
.PP

.PP
Definition at line \fB220\fP of file \fBstrevc\&.f\fP\&.
.SH "Author"
.PP 
Generated automatically by Doxygen for LAPACK from the source code\&.