ssygs2.f −

**Functions/Subroutines**

subroutine **ssygs2** (ITYPE, UPLO, N, A, LDA, B, LDB, INFO)

SSYGS2*reduces a symmetric definite generalized eigenproblem to standard form, using the factorization results obtained from spotrf (unblocked algorithm).*

* subroutine ssygs2 (integerITYPE, characterUPLO, integerN, real, dimension( lda, * )A, integerLDA, real, dimension( ldb, * )B, integerLDB, integerINFO)* reduces a symmetric definite generalized eigenproblem to standard form, using the factorization results obtained from spotrf (unblocked algorithm).

SSYGS2

**Purpose:**

SSYGS2 reduces a real symmetric-definite generalized eigenproblem

to standard form.

If ITYPE = 1, the problem is A*x = lambda*B*x,

and A is overwritten by inv(U**T)*A*inv(U) or inv(L)*A*inv(L**T)

If ITYPE = 2 or 3, the problem is A*B*x = lambda*x or

B*A*x = lambda*x, and A is overwritten by U*A*U**T or L**T *A*L.

B must have been previously factorized as U**T *U or L*L**T by SPOTRF.

**Parameters:**

*ITYPE*

ITYPE is INTEGER

= 1: compute inv(U**T)*A*inv(U) or inv(L)*A*inv(L**T);

= 2 or 3: compute U*A*U**T or L**T *A*L.

*UPLO*

UPLO is CHARACTER*1

Specifies whether the upper or lower triangular part of the

symmetric matrix A is stored, and how B has been factorized.

= ’U’: Upper triangular

= ’L’: Lower triangular

*N*

N is INTEGER

The order of the matrices A and B. N >= 0.

*A*

A is REAL array, dimension (LDA,N)

On entry, the symmetric matrix A. If UPLO = ’U’, the leading

n by n upper triangular part of A contains the upper

triangular part of the matrix A, and the strictly lower

triangular part of A is not referenced. If UPLO = ’L’, the

leading n by n lower triangular part of A contains the lower

triangular part of the matrix A, and the strictly upper

triangular part of A is not referenced.

On exit, if INFO = 0, the transformed matrix, stored in the

same format as A.

*LDA*

LDA is INTEGER

The leading dimension of the array A. LDA >= max(1,N).

*B*

B is REAL array, dimension (LDB,N)

The triangular factor from the Cholesky factorization of B,

as returned by SPOTRF.

*LDB*

LDB is INTEGER

The leading dimension of the array B. LDB >= max(1,N).

*INFO*

INFO is INTEGER

= 0: successful exit.

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

**Author:**

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

**Date:**

September 2012

Definition at line 128 of file ssygs2.f.

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