# ?geszi

Computes inverse interval matrix by Schulz iterative method.

## Syntax

call sigeszi(n, a, lda, info)

call digeszi(n, a, lda, info)

## Description

For a general interval square matrix A, the routine ?geszi computes an enclosure of the inverse interval matrix `inv(A)` by the Schulz iterative method.

See “Interval Linear Solvers Code Examples” for example code on using this routine.

## Input Parameters

n

  INTEGER. The order of the matrix A (`n ≥ 0`).

a

  REAL for sigeszi.

  DOUBLE PRECISION for digeszi.

  Array: DIMENSION (lda,*).

  Contains the matrix A.

  The second dimension of a must be at least max(1, n).

lda

  INTEGER. The first dimension of a, `lda ≥ max(1, n)`.

## Output Parameters

a

  Overwritten by an enclosure of the inverse interval matrix `inv(A)`.

info

  INTEGER.

  If `info = 0`, the execution is successful.

  If `info = i ≥ 0`, the execution is not successful.

  If `info = -i`, the i-th parameter has an illegal value.

## Application Notes

Schulz iteration implemented in ?geszi routine converges only provided that the interval matrix A is not “too wide”. Otherwise, when Schulz iteration diverges, the result is set equal to the interval matrix with the elements [ -infty,+infty ], where infty is computer infinity of the corresponding kind.

Parent topic: Routines for Inverting Interval Matrices