Pin classes exhibit a phase transition at μ ≈ 3.28277 with countably many below the threshold and uncountably many at it; all growth rates below μ are classified via periodic pin permutations.
Uncountably many enumerations of well-quasi-ordered permutation classes
1 Pith paper cite this work. Polarity classification is still indexing.
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Pith paper citing it
abstract
We construct an uncountable family of well-quasi-ordered permutation classes, each with a distinct enumeration sequence. This disproves a conjecture that all well-quasi-ordered permutation classes have algebraic generating functions, and in fact shows that many such classes lack D-finite or D-algebraic generating functions. Our construction is based on an uncountably large collection of factor-closed, well-quasi-ordered binary languages due to Pouzet.
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Pin classes II: Small pin classes
Pin classes exhibit a phase transition at μ ≈ 3.28277 with countably many below the threshold and uncountably many at it; all growth rates below μ are classified via periodic pin permutations.