CTREVC computes some or all of the right and/or left eigenvectors of


 a complex upper triangular matrix T.


 Matrices of this type are produced by the Schur factorization of


 a complex general matrix:  A = Q*T*Q**H, as computed by CHSEQR.


 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 the vector y.


 The eigenvalues are not input to this routine, but are read directly


 from the diagonal 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 unitary 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.

SIDE

          SIDE is CHARACTER*1


          = 'R':  compute right eigenvectors only;


          = 'L':  compute left eigenvectors only;


          = 'B':  compute both right and left eigenvectors.

HOWMNY

          HOWMNY is CHARACTER*1


          = 'A':  compute all right and/or left eigenvectors;


          = 'B':  compute all right and/or left eigenvectors,


                  backtransformed using the matrices supplied in


                  VR and/or VL;


          = 'S':  compute selected right and/or left eigenvectors,


                  as indicated by the logical array SELECT.

SELECT

          SELECT is LOGICAL array, dimension (N)


          If HOWMNY = 'S', SELECT specifies the eigenvectors to be


          computed.


          The eigenvector corresponding to the j-th eigenvalue is


          computed if SELECT(j) = .TRUE..


          Not referenced if HOWMNY = 'A' or 'B'.

N

          N is INTEGER


          The order of the matrix T. N >= 0.

T

          T is COMPLEX array, dimension (LDT,N)


          The upper triangular matrix T.  T is modified, but restored


          on exit.

LDT

          LDT is INTEGER


          The leading dimension of the array T. LDT >= max(1,N).

VL

          VL is COMPLEX 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 unitary matrix Q of


          Schur vectors returned by CHSEQR).


          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.


          Not referenced if SIDE = 'R'.

LDVL

          LDVL is INTEGER


          The leading dimension of the array VL.  LDVL >= 1, and if


          SIDE = 'L' or 'B', LDVL >= N.

VR

          VR is COMPLEX 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 unitary matrix Q of


          Schur vectors returned by CHSEQR).


          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.


          Not referenced if SIDE = 'L'.

LDVR

          LDVR is INTEGER


          The leading dimension of the array VR.  LDVR >= 1, and if


          SIDE = 'R' or 'B'; LDVR >= N.

MM

          MM is INTEGER


          The number of columns in the arrays VL and/or VR. MM >= M.

M

          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 eigenvector occupies one


          column.

WORK

          WORK is COMPLEX array, dimension (2*N)

RWORK

          RWORK is REAL array, dimension (N)

INFO

          INFO is INTEGER


          = 0:  successful exit


          < 0:  if INFO = -i, the i-th argument had an illegal value

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

December 2016

  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|.