cond condition number of a matrix

## Syntax

                c = cond(X)
                c = cond(X, p)

## Arguments

X

  real or complex matrix. For the `c = cond(X,p)` syntax, `X` must be square.

p

  1 | 2 | %inf | "inf" | "fro": Used norm. Default value = 2.

c

  positive decimal number: the condition number.

## Description

c = cond(X)

  returns condition number in 2-norm.`cond(X)` is the ratio of the largest singular value of `X` to the smallest.

c = cond(X, p)

  returns the condition number in p-norm : `norm(X, p) * norm(inv(X), p)`. `p` possible values are:

  1 : | condition number in 1-norm.
  2 : | condition number in 2-norm.
  %inf | "inf" : | condition number in infinite norm
  "fro" : | condition number in Frobenius norm

## Examples

    // Condition number of a non-square matrix
    cond([1 0 -1 ; 0 1 -1])

    A = testmatrix('hilb',6);
    // condition number in 2-norm
    cond(A)
    cond(A, 2)

    // condition number in 1-norm
    cond(A, 1)

    // condition number in infinity norm
    cond(A, %inf)

    // condition number in Frobenius norm
    cond(A, 'fro')

## See also

- rcond
- svd
- norm

## History

5.4.0

- cond(X) now accepts non square matrix.
- New input argument p introduced.

6.0.2 cond([]) now returns 0 instead of 1.