# ctgsja.f man page

ctgsja.f —

## Synopsis

### Functions/Subroutines

subroutinectgsja(JOBU, JOBV, JOBQ, M, P, N, K, L, A, LDA, B, LDB, TOLA, TOLB, ALPHA, BETA, U, LDU, V, LDV, Q, LDQ, WORK, NCYCLE, INFO)CTGSJA

## Function/Subroutine Documentation

### subroutine ctgsja (characterJOBU, characterJOBV, characterJOBQ, integerM, integerP, integerN, integerK, integerL, complex, dimension( lda, * )A, integerLDA, complex, dimension( ldb, * )B, integerLDB, realTOLA, realTOLB, real, dimension( * )ALPHA, real, dimension( * )BETA, complex, dimension( ldu, * )U, integerLDU, complex, dimension( ldv, * )V, integerLDV, complex, dimension( ldq, * )Q, integerLDQ, complex, dimension( * )WORK, integerNCYCLE, integerINFO)

**CTGSJA**

**Purpose:**

```
CTGSJA computes the generalized singular value decomposition (GSVD)
of two complex upper triangular (or trapezoidal) matrices A and B.
On entry, it is assumed that matrices A and B have the following
forms, which may be obtained by the preprocessing subroutine CGGSVP
from a general M-by-N matrix A and P-by-N matrix B:
N-K-L K L
A = K ( 0 A12 A13 ) if M-K-L >= 0;
L ( 0 0 A23 )
M-K-L ( 0 0 0 )
N-K-L K L
A = K ( 0 A12 A13 ) if M-K-L < 0;
M-K ( 0 0 A23 )
N-K-L K L
B = L ( 0 0 B13 )
P-L ( 0 0 0 )
where the K-by-K matrix A12 and L-by-L matrix B13 are nonsingular
upper triangular; A23 is L-by-L upper triangular if M-K-L >= 0,
otherwise A23 is (M-K)-by-L upper trapezoidal.
On exit,
U**H *A*Q = D1*( 0 R ), V**H *B*Q = D2*( 0 R ),
where U, V and Q are unitary matrices.
R is a nonsingular upper triangular matrix, and D1
and D2 are “diagonal” matrices, which are of the following
structures:
If M-K-L >= 0,
K L
D1 = K ( I 0 )
L ( 0 C )
M-K-L ( 0 0 )
K L
D2 = L ( 0 S )
P-L ( 0 0 )
N-K-L K L
( 0 R ) = K ( 0 R11 R12 ) K
L ( 0 0 R22 ) L
where
C = diag( ALPHA(K+1), ... , ALPHA(K+L) ),
S = diag( BETA(K+1), ... , BETA(K+L) ),
C**2 + S**2 = I.
R is stored in A(1:K+L,N-K-L+1:N) on exit.
If M-K-L < 0,
K M-K K+L-M
D1 = K ( I 0 0 )
M-K ( 0 C 0 )
K M-K K+L-M
D2 = M-K ( 0 S 0 )
K+L-M ( 0 0 I )
P-L ( 0 0 0 )
N-K-L K M-K K+L-M
( 0 R ) = K ( 0 R11 R12 R13 )
M-K ( 0 0 R22 R23 )
K+L-M ( 0 0 0 R33 )
where
C = diag( ALPHA(K+1), ... , ALPHA(M) ),
S = diag( BETA(K+1), ... , BETA(M) ),
C**2 + S**2 = I.
R = ( R11 R12 R13 ) is stored in A(1:M, N-K-L+1:N) and R33 is stored
( 0 R22 R23 )
in B(M-K+1:L,N+M-K-L+1:N) on exit.
The computation of the unitary transformation matrices U, V or Q
is optional. These matrices may either be formed explicitly, or they
may be postmultiplied into input matrices U1, V1, or Q1.
```

**Parameters:**

*JOBU*

```
JOBU is CHARACTER*1
= 'U': U must contain a unitary matrix U1 on entry, and
the product U1*U is returned;
= 'I': U is initialized to the unit matrix, and the
unitary matrix U is returned;
= 'N': U is not computed.
```

*JOBV*

```
JOBV is CHARACTER*1
= 'V': V must contain a unitary matrix V1 on entry, and
the product V1*V is returned;
= 'I': V is initialized to the unit matrix, and the
unitary matrix V is returned;
= 'N': V is not computed.
```

*JOBQ*

```
JOBQ is CHARACTER*1
= 'Q': Q must contain a unitary matrix Q1 on entry, and
the product Q1*Q is returned;
= 'I': Q is initialized to the unit matrix, and the
unitary matrix Q is returned;
= 'N': Q is not computed.
```

*M*

```
M is INTEGER
The number of rows of the matrix A. M >= 0.
```

*P*

```
P is INTEGER
The number of rows of the matrix B. P >= 0.
```

*N*

```
N is INTEGER
The number of columns of the matrices A and B. N >= 0.
```

*K*

`K is INTEGER`

*L*

```
L is INTEGER
K and L specify the subblocks in the input matrices A and B:
A23 = A(K+1:MIN(K+L,M),N-L+1:N) and B13 = B(1:L,,N-L+1:N)
of A and B, whose GSVD is going to be computed by CTGSJA.
See Further Details.
```

*A*

```
A is COMPLEX array, dimension (LDA,N)
On entry, the M-by-N matrix A.
On exit, A(N-K+1:N,1:MIN(K+L,M) ) contains the triangular
matrix R or part of R. See Purpose for details.
```

*LDA*

```
LDA is INTEGER
The leading dimension of the array A. LDA >= max(1,M).
```

*B*

```
B is COMPLEX array, dimension (LDB,N)
On entry, the P-by-N matrix B.
On exit, if necessary, B(M-K+1:L,N+M-K-L+1:N) contains
a part of R. See Purpose for details.
```

*LDB*

```
LDB is INTEGER
The leading dimension of the array B. LDB >= max(1,P).
```

*TOLA*

`TOLA is REAL`

*TOLB*

```
TOLB is REAL
TOLA and TOLB are the convergence criteria for the Jacobi-
Kogbetliantz iteration procedure. Generally, they are the
same as used in the preprocessing step, say
TOLA = MAX(M,N)*norm(A)*MACHEPS,
TOLB = MAX(P,N)*norm(B)*MACHEPS.
```

*ALPHA*

`ALPHA is REAL array, dimension (N)`

*BETA*

```
BETA is REAL array, dimension (N)
On exit, ALPHA and BETA contain the generalized singular
value pairs of A and B;
ALPHA(1:K) = 1,
BETA(1:K) = 0,
and if M-K-L >= 0,
ALPHA(K+1:K+L) = diag(C),
BETA(K+1:K+L) = diag(S),
or if M-K-L < 0,
ALPHA(K+1:M)= C, ALPHA(M+1:K+L)= 0
BETA(K+1:M) = S, BETA(M+1:K+L) = 1.
Furthermore, if K+L < N,
ALPHA(K+L+1:N) = 0
BETA(K+L+1:N) = 0.
```

*U*

```
U is COMPLEX array, dimension (LDU,M)
On entry, if JOBU = 'U', U must contain a matrix U1 (usually
the unitary matrix returned by CGGSVP).
On exit,
if JOBU = 'I', U contains the unitary matrix U;
if JOBU = 'U', U contains the product U1*U.
If JOBU = 'N', U is not referenced.
```

*LDU*

```
LDU is INTEGER
The leading dimension of the array U. LDU >= max(1,M) if
JOBU = 'U'; LDU >= 1 otherwise.
```

*V*

```
V is COMPLEX array, dimension (LDV,P)
On entry, if JOBV = 'V', V must contain a matrix V1 (usually
the unitary matrix returned by CGGSVP).
On exit,
if JOBV = 'I', V contains the unitary matrix V;
if JOBV = 'V', V contains the product V1*V.
If JOBV = 'N', V is not referenced.
```

*LDV*

```
LDV is INTEGER
The leading dimension of the array V. LDV >= max(1,P) if
JOBV = 'V'; LDV >= 1 otherwise.
```

*Q*

```
Q is COMPLEX array, dimension (LDQ,N)
On entry, if JOBQ = 'Q', Q must contain a matrix Q1 (usually
the unitary matrix returned by CGGSVP).
On exit,
if JOBQ = 'I', Q contains the unitary matrix Q;
if JOBQ = 'Q', Q contains the product Q1*Q.
If JOBQ = 'N', Q is not referenced.
```

*LDQ*

```
LDQ is INTEGER
The leading dimension of the array Q. LDQ >= max(1,N) if
JOBQ = 'Q'; LDQ >= 1 otherwise.
```

*WORK*

`WORK is COMPLEX array, dimension (2*N)`

*NCYCLE*

```
NCYCLE is INTEGER
The number of cycles required for convergence.
```

*INFO*

```
INFO is INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value.
= 1: the procedure does not converge after MAXIT cycles.
```

**Internal Parameters:**

```
MAXIT INTEGER
MAXIT specifies the total loops that the iterative procedure
may take. If after MAXIT cycles, the routine fails to
converge, we return INFO = 1.
```

**Author:**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Date:**

November 2011

**Further Details:**

```
CTGSJA essentially uses a variant of Kogbetliantz algorithm to reduce
min(L,M-K)-by-L triangular (or trapezoidal) matrix A23 and L-by-L
matrix B13 to the form:
U1**H *A13*Q1 = C1*R1; V1**H *B13*Q1 = S1*R1,
where U1, V1 and Q1 are unitary matrix.
C1 and S1 are diagonal matrices satisfying
C1**2 + S1**2 = I,
and R1 is an L-by-L nonsingular upper triangular matrix.
```

Definition at line 378 of file ctgsja.f.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

ctgsja(3) is an alias of ctgsja.f(3).