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<a name="Expressions-Involving-Permutation-Matrices-1"></a>
<h4 class="subsection">21.2.2 Expressions Involving Permutation Matrices</h4>
<p>If <var>P</var> is a permutation matrix and <var>M</var> a matrix, the expression
<code>P*M</code> will permute the rows of <var>M</var>. Similarly, <code>M*P</code> will
yield a column permutation.
Matrix division <code>P\M</code> and <code>M/P</code> can be used to do inverse
permutation.
</p>
<p>The previously described syntax for creating permutation matrices can actually
help an user to understand the connection between a permutation matrix and
a permuting vector. Namely, the following holds, where <code>I = eye (n)</code>
is an identity matrix:
</p>
<div class="example">
<pre class="example"> I(p,:) * M = (I*M) (p,:) = M(p,:)
</pre></div>
<p>Similarly,
</p>
<div class="example">
<pre class="example"> M * I(:,p) = (M*I) (:,p) = M(:,p)
</pre></div>
<p>The expressions <code>I(p,:)</code> and <code>I(:,p)</code> are permutation matrices.
</p>
<p>A permutation matrix can be transposed (or conjugate-transposed, which is the
same, because a permutation matrix is never complex), inverting the
permutation, or equivalently, turning a row-permutation matrix into a
column-permutation one. For permutation matrices, transpose is equivalent to
inversion, thus <code>P\M</code> is equivalent to <code>P'*M</code>. Transpose of a
permutation matrix (or inverse) is a constant-time operation, flipping only a
flag internally, and thus the choice between the two above equivalent
expressions for inverse permuting is completely up to the user’s taste.
</p>
<p>Multiplication and division by permutation matrices works efficiently also when
combined with sparse matrices, i.e., <code>P*S</code>, where <var>P</var> is a permutation
matrix and <var>S</var> is a sparse matrix permutes the rows of the sparse matrix
and returns a sparse matrix. The expressions <code>S*P</code>, <code>P\S</code>,
<code>S/P</code> work analogically.
</p>
<p>Two permutation matrices can be multiplied or divided (if their sizes match),
performing a composition of permutations. Also a permutation matrix can be
indexed by a permutation vector (or two vectors), giving again a permutation
matrix. Any other operations do not generally yield a permutation matrix and
will thus trigger the implicit conversion.
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