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{\bf M. D. Atkinson, M. M. Murphy and N. Ru\v skuc}
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{\bf Pattern Avoidance Classes and Subpermutations}
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Pattern avoidance classes of permutations that cannot be expressed as
unions of proper subclasses can be described as the set of
subpermutations of a single bijection.  In the case that this
bijection is a permutation of the natural numbers a structure theorem
is given.  The structure theorem shows that the class is almost closed
under direct sums or has a rational generating function.

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