---
name: "chegs2"
section: "3lapack"
platform: "linux"
package: "liblapack-doc"
version: "3.2.1-8"
release: "Debian 6.0 (Squeeze)"
platforms: ["linux"]
url: "https://new.carta.tech/chegs2/3lapack"
---

# chegs2(3lapack)

reduces a complex Hermitian-definite generalized eigenproblem to standard form

## NAME

CHEGS2 - reduces a complex Hermitian-definite generalized eigenproblem to standard form

## SYNOPSIS

- SUBROUTINE CHEGS2(

  ITYPE, UPLO, N, A, LDA, B, LDB, INFO )

- CHARACTER

  UPLO

- INTEGER

  INFO, ITYPE, LDA, LDB, N

- COMPLEX

  A( LDA, \* ), B( LDB, \* )

## PURPOSE

CHEGS2 reduces a complex Hermitian-definite generalized eigenproblem to standard form. If ITYPE = 1, the problem is A\*x = lambda\*B\*x,\
 and A is overwritten by inv(U')\*A\*inv(U) or inv(L)\*A\*inv(L') 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\` or L'\*A\*L. B must have been previously factorized as U'\*U or L\*L' by CPOTRF.

## ARGUMENTS

- ITYPE (input) INTEGER

  \= 1: compute inv(U')\*A\*inv(U) or inv(L)\*A\*inv(L');\
   \= 2 or 3: compute U\*A\*U' or L'\*A\*L.

- UPLO (input) CHARACTER\*1

  Specifies whether the upper or lower triangular part of the Hermitian matrix A is stored, and how B has been factorized. = 'U':  Upper triangular\
   \= 'L':  Lower triangular

- N (input) INTEGER

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

- A (input/output) COMPLEX array, dimension (LDA,N)

  On entry, the Hermitian 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 (input) INTEGER

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

- B (input) COMPLEX array, dimension (LDB,N)

  The triangular factor from the Cholesky factorization of B, as returned by CPOTRF.

- LDB (input) INTEGER

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

- INFO (output) INTEGER

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