## SYNOPSIS

- SUBROUTINE ZLATDF(
- IJOB, N, Z, LDZ, RHS, RDSUM, RDSCAL, IPIV, JPIV )

- INTEGER IJOB, LDZ, N

- DOUBLE PRECISION RDSCAL, RDSUM

- INTEGER IPIV( * ), JPIV( * )

- COMPLEX*16 RHS( * ), Z( LDZ, * )

## PURPOSE

ZLATDF computes the contribution to the reciprocal Dif-estimate by solving for x in Z * x = b, where b is chosen such that the norm of x is as large as possible. It is assumed that LU decomposition of Z has been computed by ZGETC2. On entry RHS = f holds the contribution from earlier solved sub-systems, and on return RHS = x. The factorization of Z returned by ZGETC2 has the formZ = P * L * U * Q, where P and Q are permutation matrices. L is lower triangular with unit diagonal elements and U is upper triangular.

## ARGUMENTS

- IJOB (input) INTEGER
- IJOB = 2: First compute an approximative null-vector e of Z using ZGECON, e is normalized and solve for Zx = +-e - f with the sign giving the greater value of 2-norm(x). About 5 times as expensive as Default. IJOB .ne. 2: Local look ahead strategy where all entries of the r.h.s. b is choosen as either +1 or -1. Default.
- N (input) INTEGER
- The number of columns of the matrix Z.
- Z (input) DOUBLE PRECISION array, dimension (LDZ, N)
- On entry, the LU part of the factorization of the n-by-n matrix Z computed by ZGETC2: Z = P * L * U * Q
- LDZ (input) INTEGER
- The leading dimension of the array Z. LDA >= max(1, N).
- RHS (input/output) DOUBLE PRECISION array, dimension (N).
- On entry, RHS contains contributions from other subsystems. On exit, RHS contains the solution of the subsystem with entries according to the value of IJOB (see above).
- RDSUM (input/output) DOUBLE PRECISION
- On entry, the sum of squares of computed contributions to the Dif-estimate under computation by ZTGSYL, where the scaling factor RDSCAL (see below) has been factored out. On exit, the corresponding sum of squares updated with the contributions from the current sub-system. If TRANS = 'T' RDSUM is not touched. NOTE: RDSUM only makes sense when ZTGSY2 is called by CTGSYL.
- RDSCAL (input/output) DOUBLE PRECISION
- On entry, scaling factor used to prevent overflow in RDSUM. On exit, RDSCAL is updated w.r.t. the current contributions in RDSUM. If TRANS = 'T', RDSCAL is not touched. NOTE: RDSCAL only makes sense when ZTGSY2 is called by ZTGSYL.
- IPIV (input) INTEGER array, dimension (N).
- The pivot indices; for 1 <= i <= N, row i of the matrix has been interchanged with row IPIV(i).
- JPIV (input) INTEGER array, dimension (N).
- The pivot indices; for 1 <= j <= N, column j of the matrix has been interchanged with column JPIV(j).

## FURTHER DETAILS

Based on contributions byBo Kagstrom and Peter Poromaa, Department of Computing Science,

Umea University, S-901 87 Umea, Sweden.

This routine is a further developed implementation of algorithm BSOLVE in [1] using complete pivoting in the LU factorization.

[1] Bo Kagstrom and Lars Westin,

Generalized Schur Methods with Condition Estimators for

Solving the Generalized Sylvester Equation, IEEE Transactions

on Automatic Control, Vol. 34, No. 7, July 1989, pp 745-751.

[2] Peter Poromaa,

On Efficient and Robust Estimators for the Separation

between two Regular Matrix Pairs with Applications in

Condition Estimation. Report UMINF-95.05, Department of

Computing Science, Umea University, S-901 87 Umea, Sweden,

1995.