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{\bf Ian Le}
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{\bf Wilf Classes of Pairs of Permutations of Length 4}
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$S_n(\pi_1,\pi_2,\dots, \pi_r)$ denotes the set of permutations of
length $n$ that have no subsequence with the same order relations
as any of the $\pi_i$.  In this paper we show that
$|S_n(1342,2143)|=|S_n(3142,2341)|$ and
$|S_n(1342,3124)|=|S_n(1243,2134)|$.  These two facts complete the
classification of Wilf-equivalence classes for pairs of
permutations of length four.  In both instances we exhibit
bijections between the sets using the idea of a ``block'', and in
the former we find a generating function for $|S_n(1342,2143)|$.

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