MATLAB Release Notes

The issues involved in upgrading from MATLAB 6.1 to MATLAB 6.5, in terms of mathematics features, are discussed below.

Singular Triangular Matrix Division

The result of dividing a singular lower or upper triangular matrix by any other matrix, using either left (`\`) or right (`/`) division may change. Previously, for singular square matrices `A` for which `rcond(A) = 0`, the result was always a matrix of `Inf`s.

This change is a result of performance improvements described in Mathematics Features.

Example 1.

In MATLAB Version 6.5,

• ```A = [1 2 3;0 4 5;0 0 0];
b = [1;2;3];
A\b
Warning: Matrix is close to singular or badly scaled.
Results may be inaccurate. RCOND = 0.000000e+000.

ans =
NaN
-Inf
Inf
```

Previously, the result was

• ```[Inf
Inf
Inf]
```

Example 2.

In MATLAB 6.5, a zero matrix is treated as a singular triangular matrix.

• ```[0 0;0 0] \ [0 0]'
Warning: Matrix is close to singular or badly scaled.
Results may be inaccurate. RCOND = 0.000000e+000.

ans =
NaN
NaN
```

Previously the result was

• ```[Inf
Inf]
```