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The package \spadtype{Permanent} provides the function
\spadfunFrom{permanent}{Permanent} for square matrices.
\index{matrix!permanent of}
The \spadfunFrom{permanent}{Permanent} of a square matrix can be computed
in the same way as the determinant by expansion of minors except that for
the permanent the sign for each element is \spad{1}, rather than being
\spad{1} if the row plus column indices is positive and \spad{-1}
otherwise.
This function is much more difficult to compute efficiently than the
\spadfunFrom{determinant}{Matrix}.
An example of the use of \spadfunFrom{permanent}{Permanent} is the
calculation of the \eth{n} derangement number, defined to be the number
of different possibilities for \spad{n} couples to dance but never with
their own spouse.
\xtc{
Consider an \spad{n} by \spad{n} matrix with entries \spad{0} on the diagonal and
\spad{1} elsewhere.
Think of the rows as one-half of each couple (for example, the males) and the
columns as the other half.
The permanent of such a matrix gives the desired derangement number.
}{
\begin{spadsrc}[\bound{kn}]
kn n ==
  r : MATRIX INT := new(n,n,1)
  for i in 1..n repeat
      r(i, i) := 0
  r
\end{spadsrc}
}
\xtc{
Here are some derangement numbers, which you see grow quite fast.
}{
\spadcommand{permanent(kn(5) :: SQMATRIX(5,INT)) \free{kn}}
}
\xtc{
}{
\spadcommand{[permanent(kn(n) :: SQMATRIX(n,INT)) for n in 1..13] \free{kn}}
}
