LAPACK
3.4.2
LAPACK: Linear Algebra PACKage

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Functions/Subroutines  
subroutine  ctgsen (IJOB, WANTQ, WANTZ, SELECT, N, A, LDA, B, LDB, ALPHA, BETA, Q, LDQ, Z, LDZ, M, PL, PR, DIF, WORK, LWORK, IWORK, LIWORK, INFO) 
CTGSEN 
subroutine ctgsen  (  integer  IJOB, 
logical  WANTQ,  
logical  WANTZ,  
logical, dimension( * )  SELECT,  
integer  N,  
complex, dimension( lda, * )  A,  
integer  LDA,  
complex, dimension( ldb, * )  B,  
integer  LDB,  
complex, dimension( * )  ALPHA,  
complex, dimension( * )  BETA,  
complex, dimension( ldq, * )  Q,  
integer  LDQ,  
complex, dimension( ldz, * )  Z,  
integer  LDZ,  
integer  M,  
real  PL,  
real  PR,  
real, dimension( * )  DIF,  
complex, dimension( * )  WORK,  
integer  LWORK,  
integer, dimension( * )  IWORK,  
integer  LIWORK,  
integer  INFO  
) 
CTGSEN
Download CTGSEN + dependencies [TGZ] [ZIP] [TXT]CTGSEN reorders the generalized Schur decomposition of a complex matrix pair (A, B) (in terms of an unitary equivalence trans formation Q**H * (A, B) * Z), so that a selected cluster of eigenvalues appears in the leading diagonal blocks of the pair (A,B). The leading columns of Q and Z form unitary bases of the corresponding left and right eigenspaces (deflating subspaces). (A, B) must be in generalized Schur canonical form, that is, A and B are both upper triangular. CTGSEN also computes the generalized eigenvalues w(j)= ALPHA(j) / BETA(j) of the reordered matrix pair (A, B). Optionally, the routine computes estimates of reciprocal condition numbers for eigenvalues and eigenspaces. These are Difu[(A11,B11), (A22,B22)] and Difl[(A11,B11), (A22,B22)], i.e. the separation(s) between the matrix pairs (A11, B11) and (A22,B22) that correspond to the selected cluster and the eigenvalues outside the cluster, resp., and norms of "projections" onto left and right eigenspaces w.r.t. the selected cluster in the (1,1)block.
[in]  IJOB  IJOB is integer Specifies whether condition numbers are required for the cluster of eigenvalues (PL and PR) or the deflating subspaces (Difu and Difl): =0: Only reorder w.r.t. SELECT. No extras. =1: Reciprocal of norms of "projections" onto left and right eigenspaces w.r.t. the selected cluster (PL and PR). =2: Upper bounds on Difu and Difl. Fnormbased estimate (DIF(1:2)). =3: Estimate of Difu and Difl. 1normbased estimate (DIF(1:2)). About 5 times as expensive as IJOB = 2. =4: Compute PL, PR and DIF (i.e. 0, 1 and 2 above): Economic version to get it all. =5: Compute PL, PR and DIF (i.e. 0, 1 and 3 above) 
[in]  WANTQ  WANTQ is LOGICAL .TRUE. : update the left transformation matrix Q; .FALSE.: do not update Q. 
[in]  WANTZ  WANTZ is LOGICAL .TRUE. : update the right transformation matrix Z; .FALSE.: do not update Z. 
[in]  SELECT  SELECT is LOGICAL array, dimension (N) SELECT specifies the eigenvalues in the selected cluster. To select an eigenvalue w(j), SELECT(j) must be set to .TRUE.. 
[in]  N  N is INTEGER The order of the matrices A and B. N >= 0. 
[in,out]  A  A is COMPLEX array, dimension(LDA,N) On entry, the upper triangular matrix A, in generalized Schur canonical form. On exit, A is overwritten by the reordered matrix A. 
[in]  LDA  LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N). 
[in,out]  B  B is COMPLEX array, dimension(LDB,N) On entry, the upper triangular matrix B, in generalized Schur canonical form. On exit, B is overwritten by the reordered matrix B. 
[in]  LDB  LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N). 
[out]  ALPHA  ALPHA is COMPLEX array, dimension (N) 
[out]  BETA  BETA is COMPLEX array, dimension (N) The diagonal elements of A and B, respectively, when the pair (A,B) has been reduced to generalized Schur form. ALPHA(i)/BETA(i) i=1,...,N are the generalized eigenvalues. 
[in,out]  Q  Q is COMPLEX array, dimension (LDQ,N) On entry, if WANTQ = .TRUE., Q is an NbyN matrix. On exit, Q has been postmultiplied by the left unitary transformation matrix which reorder (A, B); The leading M columns of Q form orthonormal bases for the specified pair of left eigenspaces (deflating subspaces). If WANTQ = .FALSE., Q is not referenced. 
[in]  LDQ  LDQ is INTEGER The leading dimension of the array Q. LDQ >= 1. If WANTQ = .TRUE., LDQ >= N. 
[in,out]  Z  Z is COMPLEX array, dimension (LDZ,N) On entry, if WANTZ = .TRUE., Z is an NbyN matrix. On exit, Z has been postmultiplied by the left unitary transformation matrix which reorder (A, B); The leading M columns of Z form orthonormal bases for the specified pair of left eigenspaces (deflating subspaces). If WANTZ = .FALSE., Z is not referenced. 
[in]  LDZ  LDZ is INTEGER The leading dimension of the array Z. LDZ >= 1. If WANTZ = .TRUE., LDZ >= N. 
[out]  M  M is INTEGER The dimension of the specified pair of left and right eigenspaces, (deflating subspaces) 0 <= M <= N. 
[out]  PL  PL is REAL 
[out]  PR  PR is REAL If IJOB = 1, 4 or 5, PL, PR are lower bounds on the reciprocal of the norm of "projections" onto left and right eigenspace with respect to the selected cluster. 0 < PL, PR <= 1. If M = 0 or M = N, PL = PR = 1. If IJOB = 0, 2 or 3 PL, PR are not referenced. 
[out]  DIF  DIF is REAL array, dimension (2). If IJOB >= 2, DIF(1:2) store the estimates of Difu and Difl. If IJOB = 2 or 4, DIF(1:2) are Fnormbased upper bounds on Difu and Difl. If IJOB = 3 or 5, DIF(1:2) are 1normbased estimates of Difu and Difl, computed using reversed communication with CLACN2. If M = 0 or N, DIF(1:2) = Fnorm([A, B]). If IJOB = 0 or 1, DIF is not referenced. 
[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 >= 1 If IJOB = 1, 2 or 4, LWORK >= 2*M*(NM) If IJOB = 3 or 5, LWORK >= 4*M*(NM) If LWORK = 1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA. 
[out]  IWORK  IWORK is INTEGER array, dimension (MAX(1,LIWORK)) On exit, if INFO = 0, IWORK(1) returns the optimal LIWORK. 
[in]  LIWORK  LIWORK is INTEGER The dimension of the array IWORK. LIWORK >= 1. If IJOB = 1, 2 or 4, LIWORK >= N+2; If IJOB = 3 or 5, LIWORK >= MAX(N+2, 2*M*(NM)); If LIWORK = 1, then a workspace query is assumed; the routine only calculates the optimal size of the IWORK array, returns this value as the first entry of the IWORK array, and no error message related to LIWORK is issued by XERBLA. 
[out]  INFO  INFO is INTEGER =0: Successful exit. <0: If INFO = i, the ith argument had an illegal value. =1: Reordering of (A, B) failed because the transformed matrix pair (A, B) would be too far from generalized Schur form; the problem is very illconditioned. (A, B) may have been partially reordered. If requested, 0 is returned in DIF(*), PL and PR. 
CTGSEN first collects the selected eigenvalues by computing unitary U and W that move them to the top left corner of (A, B). In other words, the selected eigenvalues are the eigenvalues of (A11, B11) in U**H*(A, B)*W = (A11 A12) (B11 B12) n1 ( 0 A22),( 0 B22) n2 n1 n2 n1 n2 where N = n1+n2 and U**H means the conjugate transpose of U. The first n1 columns of U and W span the specified pair of left and right eigenspaces (deflating subspaces) of (A, B). If (A, B) has been obtained from the generalized real Schur decomposition of a matrix pair (C, D) = Q*(A, B)*Z', then the reordered generalized Schur form of (C, D) is given by (C, D) = (Q*U)*(U**H *(A, B)*W)*(Z*W)**H, and the first n1 columns of Q*U and Z*W span the corresponding deflating subspaces of (C, D) (Q and Z store Q*U and Z*W, resp.). Note that if the selected eigenvalue is sufficiently illconditioned, then its value may differ significantly from its value before reordering. The reciprocal condition numbers of the left and right eigenspaces spanned by the first n1 columns of U and W (or Q*U and Z*W) may be returned in DIF(1:2), corresponding to Difu and Difl, resp. The Difu and Difl are defined as: Difu[(A11, B11), (A22, B22)] = sigmamin( Zu ) and Difl[(A11, B11), (A22, B22)] = Difu[(A22, B22), (A11, B11)], where sigmamin(Zu) is the smallest singular value of the (2*n1*n2)by(2*n1*n2) matrix Zu = [ kron(In2, A11) kron(A22**H, In1) ] [ kron(In2, B11) kron(B22**H, In1) ]. Here, Inx is the identity matrix of size nx and A22**H is the conjuguate transpose of A22. kron(X, Y) is the Kronecker product between the matrices X and Y. When DIF(2) is small, small changes in (A, B) can cause large changes in the deflating subspace. An approximate (asymptotic) bound on the maximum angular error in the computed deflating subspaces is EPS * norm((A, B)) / DIF(2), where EPS is the machine precision. The reciprocal norm of the projectors on the left and right eigenspaces associated with (A11, B11) may be returned in PL and PR. They are computed as follows. First we compute L and R so that P*(A, B)*Q is block diagonal, where P = ( I L ) n1 Q = ( I R ) n1 ( 0 I ) n2 and ( 0 I ) n2 n1 n2 n1 n2 and (L, R) is the solution to the generalized Sylvester equation A11*R  L*A22 = A12 B11*R  L*B22 = B12 Then PL = (Fnorm(L)**2+1)**(1/2) and PR = (Fnorm(R)**2+1)**(1/2). An approximate (asymptotic) bound on the average absolute error of the selected eigenvalues is EPS * norm((A, B)) / PL. There are also global error bounds which valid for perturbations up to a certain restriction: A lower bound (x) on the smallest Fnorm(E,F) for which an eigenvalue of (A11, B11) may move and coalesce with an eigenvalue of (A22, B22) under perturbation (E,F), (i.e. (A + E, B + F), is x = min(Difu,Difl)/((1/(PL*PL)+1/(PR*PR))**(1/2)+2*max(1/PL,1/PR)). An approximate bound on x can be computed from DIF(1:2), PL and PR. If y = ( Fnorm(E,F) / x) <= 1, the angles between the perturbed (L', R') and unperturbed (L, R) left and right deflating subspaces associated with the selected cluster in the (1,1)blocks can be bounded as maxangle(L, L') <= arctan( y * PL / (1  y * (1  PL * PL)**(1/2)) maxangle(R, R') <= arctan( y * PR / (1  y * (1  PR * PR)**(1/2)) See LAPACK User's Guide section 4.11 or the following references for more information. Note that if the default method for computing the Frobeniusnorm based estimate DIF is not wanted (see CLATDF), then the parameter IDIFJB (see below) should be changed from 3 to 4 (routine CLATDF (IJOB = 2 will be used)). See CTGSYL for more details.
Definition at line 432 of file ctgsen.f.