  The permutation language $P_n$ consists of all words that are permutations of
a fixed alphabet of size $n$. Using divide-and-conquer, we construct a regular
expression $R_n$ that specifies $P_n$. We then give explicit bounds for the
length of $R_n$, which we find to be $4^n n^{-(\lg n)/4+\Theta(1)}$, and use
these bounds to show that $R_n$ has minimum size over all regular expressions
specifying $P_n$.
