cgehd2 (l)  Linux Manuals
cgehd2: reduces a complex general matrix A to upper Hessenberg form H by a unitary similarity transformation
NAME
CGEHD2  reduces a complex general matrix A to upper Hessenberg form H by a unitary similarity transformationSYNOPSIS
 SUBROUTINE CGEHD2(
 N, ILO, IHI, A, LDA, TAU, WORK, INFO )
 INTEGER IHI, ILO, INFO, LDA, N
 COMPLEX A( LDA, * ), TAU( * ), WORK( * )
PURPOSE
CGEHD2 reduces a complex general matrix A to upper Hessenberg form H by a unitary similarity transformation: Qaq * A * Q = H .ARGUMENTS
 N (input) INTEGER
 The order of the matrix A. N >= 0.
 ILO (input) INTEGER
 IHI (input) INTEGER It is assumed that A is already upper triangular in rows and columns 1:ILO1 and IHI+1:N. ILO and IHI are normally set by a previous call to CGEBAL; otherwise they should be set to 1 and N respectively. See Further Details.
 A (input/output) COMPLEX array, dimension (LDA,N)
 On entry, the n by n general matrix to be reduced. On exit, the upper triangle and the first subdiagonal of A are overwritten with the upper Hessenberg matrix H, and the elements below the first subdiagonal, with the array TAU, represent the unitary matrix Q as a product of elementary reflectors. See Further Details. LDA (input) INTEGER The leading dimension of the array A. LDA >= max(1,N).
 TAU (output) COMPLEX array, dimension (N1)
 The scalar factors of the elementary reflectors (see Further Details).
 WORK (workspace) COMPLEX array, dimension (N)
 INFO (output) INTEGER

= 0: successful exit
< 0: if INFO = i, the ith argument had an illegal value.
FURTHER DETAILS
The matrix Q is represented as a product of (ihiilo) elementary reflectorsQ
Each H(i) has the form
H(i)
where tau is a complex scalar, and v is a complex vector with v(1:i) = 0, v(i+1) = 1 and v(ihi+1:n) = 0; v(i+2:ihi) is stored on exit in A(i+2:ihi,i), and tau in TAU(i).
The contents of A are illustrated by the following example, with n = 7, ilo = 2 and ihi = 6:
on entry, on exit,
( a a a a a a a ) ( a a h h h h a ) ( a a a a a a ) ( a h h h h a ) ( a a a a a a ) ( h h h h h h ) ( a a a a a a ) ( v2 h h h h h ) ( a a a a a a ) ( v2 v3 h h h h ) ( a a a a a a ) ( v2 v3 v4 h h h ) ( a ) ( a ) where a denotes an element of the original matrix A, h denotes a modified element of the upper Hessenberg matrix H, and vi denotes an element of the vector defining H(i).