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<h4 class="subsection">20.3.2 Permutation Matrix Functions</h4>
<p><dfn>inv</dfn> and <dfn>pinv</dfn> will invert a permutation matrix, preserving its
specialness. <dfn>det</dfn> can be applied to a permutation matrix, efficiently
calculating the sign of the permutation (which is equal to the determinant).
<p>A permutation matrix can also be returned from the built-in functions
<dfn>lu</dfn> and <dfn>qr</dfn>, if a pivoted factorization is requested.
<p>The <dfn>sparse</dfn> function will convert a permutation matrix efficiently to a
sparse matrix.
The <dfn>find</dfn> function will also work efficiently with a permutation matrix,
making it possible to conveniently obtain the permutation indices.
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