
arXiv: 1106.3653
We study questions of even-Wilf-equivalence, the analogue of Wilf-equivalence when attention is restricted to pattern avoidance by permutations in the alternating group. Although some Wilf-equivalence results break when considering even-Wilf-equivalence analogues, we prove that other Wilf-equivalence results continue to hold in the even-Wilf-equivalence setting. In particular, we prove that $t(t-1)\cdots 321$ and $(t-1)(t-2)\cdots 21t$ are even-shape-Wilf-equivalent for odd $t$, paralleling a result (which held for all $t$) of Backelin, West, and Xin for shape-Wilf-equivalence. This allows us to classify the symmetric group $\mathcal{S}_{4}$, and to partially classify $\mathcal{S}_{5}$ and $\mathcal{S}_{6}$, according to even-Wilf-equivalence. As with transition to involution-Wilf-equivalence, some—but not all—of the classical Wilf-equivalence results are preserved when we make the transition to even-Wilf-equivalence.
Permutations, words, matrices, Wilf-equivalence, Exact enumeration problems, generating functions, even permutation, even-Wilf-equivalence, 05A05, FOS: Mathematics, Mathematics - Combinatorics, Combinatorics (math.CO), permutation pattern, alternating group, Combinatorial identities, bijective combinatorics, Combinatorial aspects of matrices (incidence, Hadamard, etc.)
Permutations, words, matrices, Wilf-equivalence, Exact enumeration problems, generating functions, even permutation, even-Wilf-equivalence, 05A05, FOS: Mathematics, Mathematics - Combinatorics, Combinatorics (math.CO), permutation pattern, alternating group, Combinatorial identities, bijective combinatorics, Combinatorial aspects of matrices (incidence, Hadamard, etc.)
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