# sgegs.f - Man Page

SRC/DEPRECATED/sgegs.f

## Synopsis

### Functions/Subroutines

subroutine **sgegs** (jobvsl, jobvsr, n, a, lda, b, ldb, alphar, alphai, beta, vsl, ldvsl, vsr, ldvsr, work, lwork, info)

**SGEGS computes the eigenvalues, real Schur form, and, optionally, the left and/or right Schur vectors of a real matrix pair (A,B)**

## Function/Subroutine Documentation

### subroutine sgegs (character jobvsl, character jobvsr, integer n, real, dimension( lda, * ) a, integer lda, real, dimension( ldb, * ) b, integer ldb, real, dimension( * ) alphar, real, dimension( * ) alphai, real, dimension( * ) beta, real, dimension( ldvsl, * ) vsl, integer ldvsl, real, dimension( ldvsr, * ) vsr, integer ldvsr, real, dimension( * ) work, integer lwork, integer info)

**SGEGS computes the eigenvalues, real Schur form, and, optionally, the left and/or right Schur vectors of a real matrix pair (A,B)**

**Purpose:**

This routine is deprecated and has been replaced by routine SGGES. SGEGS computes the eigenvalues, real Schur form, and, optionally, left and or/right Schur vectors of a real matrix pair (A,B). Given two square matrices A and B, the generalized real Schur factorization has the form A = Q*S*Z**T, B = Q*T*Z**T where Q and Z are orthogonal matrices, T is upper triangular, and S is an upper quasi-triangular matrix with 1-by-1 and 2-by-2 diagonal blocks, the 2-by-2 blocks corresponding to complex conjugate pairs of eigenvalues of (A,B). The columns of Q are the left Schur vectors and the columns of Z are the right Schur vectors. If only the eigenvalues of (A,B) are needed, the driver routine SGEGV should be used instead. See SGEGV for a description of the eigenvalues of the generalized nonsymmetric eigenvalue problem (GNEP).

**Parameters***JOBVSL*JOBVSL is CHARACTER*1 = 'N': do not compute the left Schur vectors; = 'V': compute the left Schur vectors (returned in VSL).

*JOBVSR*JOBVSR is CHARACTER*1 = 'N': do not compute the right Schur vectors; = 'V': compute the right Schur vectors (returned in VSR).

*N*N is INTEGER The order of the matrices A, B, VSL, and VSR. N >= 0.

*A*A is REAL array, dimension (LDA, N) On entry, the matrix A. On exit, the upper quasi-triangular matrix S from the generalized real Schur factorization.

*LDA*LDA is INTEGER The leading dimension of A. LDA >= max(1,N).

*B*B is REAL array, dimension (LDB, N) On entry, the matrix B. On exit, the upper triangular matrix T from the generalized real Schur factorization.

*LDB*LDB is INTEGER The leading dimension of B. LDB >= max(1,N).

*ALPHAR*ALPHAR is REAL array, dimension (N) The real parts of each scalar alpha defining an eigenvalue of GNEP.

*ALPHAI*ALPHAI is REAL array, dimension (N) The imaginary parts of each scalar alpha defining an eigenvalue of GNEP. If ALPHAI(j) is zero, then the j-th eigenvalue is real; if positive, then the j-th and (j+1)-st eigenvalues are a complex conjugate pair, with ALPHAI(j+1) = -ALPHAI(j).

*BETA*BETA is REAL array, dimension (N) The scalars beta that define the eigenvalues of GNEP. Together, the quantities alpha = (ALPHAR(j),ALPHAI(j)) and beta = BETA(j) represent the j-th eigenvalue of the matrix pair (A,B), in one of the forms lambda = alpha/beta or mu = beta/alpha. Since either lambda or mu may overflow, they should not, in general, be computed.

*VSL*VSL is REAL array, dimension (LDVSL,N) If JOBVSL = 'V', the matrix of left Schur vectors Q. Not referenced if JOBVSL = 'N'.

*LDVSL*LDVSL is INTEGER The leading dimension of the matrix VSL. LDVSL >=1, and if JOBVSL = 'V', LDVSL >= N.

*VSR*VSR is REAL array, dimension (LDVSR,N) If JOBVSR = 'V', the matrix of right Schur vectors Z. Not referenced if JOBVSR = 'N'.

*LDVSR*LDVSR is INTEGER The leading dimension of the matrix VSR. LDVSR >= 1, and if JOBVSR = 'V', LDVSR >= N.

*WORK*WORK is REAL array, dimension (MAX(1,LWORK)) On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

*LWORK*LWORK is INTEGER The dimension of the array WORK. LWORK >= max(1,4*N). For good performance, LWORK must generally be larger. To compute the optimal value of LWORK, call ILAENV to get blocksizes (for SGEQRF, SORMQR, and SORGQR.) Then compute: NB -- MAX of the blocksizes for SGEQRF, SORMQR, and SORGQR The optimal LWORK is 2*N + N*(NB+1). 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.

*INFO*INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value. = 1,...,N: The QZ iteration failed. (A,B) are not in Schur form, but ALPHAR(j), ALPHAI(j), and BETA(j) should be correct for j=INFO+1,...,N. > N: errors that usually indicate LAPACK problems: =N+1: error return from SGGBAL =N+2: error return from SGEQRF =N+3: error return from SORMQR =N+4: error return from SORGQR =N+5: error return from SGGHRD =N+6: error return from SHGEQZ (other than failed iteration) =N+7: error return from SGGBAK (computing VSL) =N+8: error return from SGGBAK (computing VSR) =N+9: error return from SLASCL (various places)

**Author**Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line **224** of file **sgegs.f**.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page sgegs(3) is an alias of sgegs.f(3).

Tue Nov 28 2023 12:08:41 Version 3.12.0 LAPACK