isIllConditioned

Determine whether matrix is ill conditioned

Syntax

tf = isIllConditioned(dA)

Description

tf = isIllConditioned(dA) returns logical 1 (true) if the original coefficient matrix A used to create decomposition dA is ill conditioned; otherwise, it returns logical 0 (false).

The test used depends on the type of decomposition:

• 'qr' and 'cod' decompositions — The coefficient matrix is ill conditioned if rank(dA) < min(size(A)).

• All other decompositions — The coefficient matrix is ill conditioned if rcond(dA) < eps.

If isIllConditioned returns logical 1 (true), then solving a linear system with either dA\b or b/dA displays a warning. Use the CheckCondition property of the decomposition object dA to turn off these warnings.

Examples

Check Condition of Coefficient Matrix

Create a matrix decomposition object for a 25-by-25 Hilbert coefficient matrix and then check to see whether the underlying coefficient matrix is ill conditioned.

A = hilb(25);
dA = decomposition(A)

dA = 
  decomposition with properties:

    MatrixSize: [25 25]
          Type: 'lu'

  Show all properties

tf = isIllConditioned(dA)

tf = logical
   1

Check the reciprocal condition number of the coefficient matrix. In this case isIllConditioned determines that the coefficient matrix A is ill conditioned because rcond(dA) is smaller than eps.

rcond(dA)

ans = 
1.9416e-19

Input Arguments

dA — Input decomposition

decomposition object

Input decomposition, specified as a decomposition object.

Example: dA = decomposition(A,'qr')

Tips

• isIllConditioned uses rank and condition number estimates of the decomposition object. These estimates can differ compared to calling rank(A) or rcond(A) on the coefficient matrix directly.

Version History

Introduced in R2017b