LAPACK
3.4.2
LAPACK: Linear Algebra PACKage

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Functions/Subroutines  
subroutine  ctgsna (JOB, HOWMNY, SELECT, N, A, LDA, B, LDB, VL, LDVL, VR, LDVR, S, DIF, MM, M, WORK, LWORK, IWORK, INFO) 
CTGSNA 
subroutine ctgsna  (  character  JOB, 
character  HOWMNY,  
logical, dimension( * )  SELECT,  
integer  N,  
complex, dimension( lda, * )  A,  
integer  LDA,  
complex, dimension( ldb, * )  B,  
integer  LDB,  
complex, dimension( ldvl, * )  VL,  
integer  LDVL,  
complex, dimension( ldvr, * )  VR,  
integer  LDVR,  
real, dimension( * )  S,  
real, dimension( * )  DIF,  
integer  MM,  
integer  M,  
complex, dimension( * )  WORK,  
integer  LWORK,  
integer, dimension( * )  IWORK,  
integer  INFO  
) 
CTGSNA
Download CTGSNA + dependencies [TGZ] [ZIP] [TXT]CTGSNA estimates reciprocal condition numbers for specified eigenvalues and/or eigenvectors of a matrix pair (A, B). (A, B) must be in generalized Schur canonical form, that is, A and B are both upper triangular.
[in]  JOB  JOB is CHARACTER*1 Specifies whether condition numbers are required for eigenvalues (S) or eigenvectors (DIF): = 'E': for eigenvalues only (S); = 'V': for eigenvectors only (DIF); = 'B': for both eigenvalues and eigenvectors (S and DIF). 
[in]  HOWMNY  HOWMNY is CHARACTER*1 = 'A': compute condition numbers for all eigenpairs; = 'S': compute condition numbers for selected eigenpairs specified by the array SELECT. 
[in]  SELECT  SELECT is LOGICAL array, dimension (N) If HOWMNY = 'S', SELECT specifies the eigenpairs for which condition numbers are required. To select condition numbers for the corresponding jth eigenvalue and/or eigenvector, SELECT(j) must be set to .TRUE.. If HOWMNY = 'A', SELECT is not referenced. 
[in]  N  N is INTEGER The order of the square matrix pair (A, B). N >= 0. 
[in]  A  A is COMPLEX array, dimension (LDA,N) The upper triangular matrix A in the pair (A,B). 
[in]  LDA  LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N). 
[in]  B  B is COMPLEX array, dimension (LDB,N) The upper triangular matrix B in the pair (A, B). 
[in]  LDB  LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N). 
[in]  VL  VL is COMPLEX array, dimension (LDVL,M) IF JOB = 'E' or 'B', VL must contain left eigenvectors of (A, B), corresponding to the eigenpairs specified by HOWMNY and SELECT. The eigenvectors must be stored in consecutive columns of VL, as returned by CTGEVC. If JOB = 'V', VL is not referenced. 
[in]  LDVL  LDVL is INTEGER The leading dimension of the array VL. LDVL >= 1; and If JOB = 'E' or 'B', LDVL >= N. 
[in]  VR  VR is COMPLEX array, dimension (LDVR,M) IF JOB = 'E' or 'B', VR must contain right eigenvectors of (A, B), corresponding to the eigenpairs specified by HOWMNY and SELECT. The eigenvectors must be stored in consecutive columns of VR, as returned by CTGEVC. If JOB = 'V', VR is not referenced. 
[in]  LDVR  LDVR is INTEGER The leading dimension of the array VR. LDVR >= 1; If JOB = 'E' or 'B', LDVR >= N. 
[out]  S  S is REAL array, dimension (MM) If JOB = 'E' or 'B', the reciprocal condition numbers of the selected eigenvalues, stored in consecutive elements of the array. If JOB = 'V', S is not referenced. 
[out]  DIF  DIF is REAL array, dimension (MM) If JOB = 'V' or 'B', the estimated reciprocal condition numbers of the selected eigenvectors, stored in consecutive elements of the array. If the eigenvalues cannot be reordered to compute DIF(j), DIF(j) is set to 0; this can only occur when the true value would be very small anyway. For each eigenvalue/vector specified by SELECT, DIF stores a Frobenius normbased estimate of Difl. If JOB = 'E', DIF is not referenced. 
[in]  MM  MM is INTEGER The number of elements in the arrays S and DIF. MM >= M. 
[out]  M  M is INTEGER The number of elements of the arrays S and DIF used to store the specified condition numbers; for each selected eigenvalue one element is used. If HOWMNY = 'A', M is set to N. 
[out]  WORK  WORK is COMPLEX array, dimension (MAX(1,LWORK)) On exit, if INFO = 0, WORK(1) returns the optimal LWORK. 
[in]  LWORK  LWORK is INTEGER The dimension of the array WORK. LWORK >= max(1,N). If JOB = 'V' or 'B', LWORK >= max(1,2*N*N). 
[out]  IWORK  IWORK is INTEGER array, dimension (N+2) If JOB = 'E', IWORK is not referenced. 
[out]  INFO  INFO is INTEGER = 0: Successful exit < 0: If INFO = i, the ith argument had an illegal value 
The reciprocal of the condition number of the ith generalized eigenvalue w = (a, b) is defined as S(I) = (v**HAu**2 + v**HBu**2)**(1/2) / (norm(u)*norm(v)) where u and v are the right and left eigenvectors of (A, B) corresponding to w; z denotes the absolute value of the complex number, and norm(u) denotes the 2norm of the vector u. The pair (a, b) corresponds to an eigenvalue w = a/b (= v**HAu/v**HBu) of the matrix pair (A, B). If both a and b equal zero, then (A,B) is singular and S(I) = 1 is returned. An approximate error bound on the chordal distance between the ith computed generalized eigenvalue w and the corresponding exact eigenvalue lambda is chord(w, lambda) <= EPS * norm(A, B) / S(I), where EPS is the machine precision. The reciprocal of the condition number of the right eigenvector u and left eigenvector v corresponding to the generalized eigenvalue w is defined as follows. Suppose (A, B) = ( a * ) ( b * ) 1 ( 0 A22 ),( 0 B22 ) n1 1 n1 1 n1 Then the reciprocal condition number DIF(I) is Difl[(a, b), (A22, B22)] = sigmamin( Zl ) where sigmamin(Zl) denotes the smallest singular value of Zl = [ kron(a, In1) kron(1, A22) ] [ kron(b, In1) kron(1, B22) ]. Here In1 is the identity matrix of size n1 and X**H is the conjugate transpose of X. kron(X, Y) is the Kronecker product between the matrices X and Y. We approximate the smallest singular value of Zl with an upper bound. This is done by CLATDF. An approximate error bound for a computed eigenvector VL(i) or VR(i) is given by EPS * norm(A, B) / DIF(i). See ref. [23] for more details and further references.
[1] B. Kagstrom; A Direct Method for Reordering Eigenvalues in the Generalized Real Schur Form of a Regular Matrix Pair (A, B), in M.S. Moonen et al (eds), Linear Algebra for Large Scale and RealTime Applications, Kluwer Academic Publ. 1993, pp 195218. [2] B. Kagstrom and P. Poromaa; Computing Eigenspaces with Specified Eigenvalues of a Regular Matrix Pair (A, B) and Condition Estimation: Theory, Algorithms and Software, Report UMINF  94.04, Department of Computing Science, Umea University, S901 87 Umea, Sweden, 1994. Also as LAPACK Working Note 87. To appear in Numerical Algorithms, 1996. [3] B. Kagstrom and P. Poromaa, LAPACKStyle Algorithms and Software for Solving the Generalized Sylvester Equation and Estimating the Separation between Regular Matrix Pairs, Report UMINF  93.23, Department of Computing Science, Umea University, S901 87 Umea, Sweden, December 1993, Revised April 1994, Also as LAPACK Working Note 75. To appear in ACM Trans. on Math. Software, Vol 22, No 1, 1996.
Definition at line 310 of file ctgsna.f.