Frame patterns in n-cycles

In this paper, we study the distribution of the number of occurrences of the simplest frame pattern, called the $μ$ pattern, in $n$-cycles. Given an $n$-cycle $C$, we say that a pair $\langle i,j \rangle$ matches the $μ$ pattern if $i < j$ and as we traverse around $C$ in a clockwise direction starting at $i$ and ending at $j$, we never encounter a $k$ with $i < k < j$. We say that $\langle i,j \rangle$ is a nontrivial $μ$-match if $i+1 < j$. Also, an $n$-cycle $C$ is incontractible if there is no $i$ such that $i+1$ immediately follows $i$ in $C$. We show that the number of incontractible $n$-cycles in the symmetric group $S_n$ is $D_{n-1}$, where $D_n$ is the number of derangements in $S_n$. Further, we prove that the number of $n$-cycles in $S_n$ with exactly $k$ $μ$-matches can be expressed as a linear combination of binomial coefficients of the form $\binom{n-1}{i}$ where $i \leq 2k+1$. We also show that the generating function $NTI_{n,μ}(q)$ of $q$ raised to the number of nontrivial $μ$-matches in $C$ over all incontractible $n$-cycles in $S_n$ is a new $q$-analogue of $D_{n-1}$, which is different from the $q$-analogues of the derangement numbers that have been studied by Garsia and Remmel and by Wachs. We show that there is a rather surprising connection between the charge statistic on permutations due to Lascoux and Schüzenberger and our polynomials in that the coefficient of the smallest power of $q$ in $NTI_{2k+1,μ}(q)$ is the number of permutations in $S_{2k+1}$ whose charge path is a Dyck path. Finally, we show that $NTI_{n,μ}(q)|_{q^{\binom{n-1}{2} -k}}$ and $NT_{n,μ}(q)|_{q^{\binom{n-1}{2} -k}}$ are the number of partitions of $k$ for sufficiently large $n$.

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