Restricted Permutations, Fibonacci Numbers, and k-generalized Fibonacci Numbers

Abstract

A permutation $\pi \in S_n$ is said to {\it avoid} a permutation $\sigma \in S_k$ whenever $\pi$ contains no subsequence with all of the same pairwise comparisons as $\sigma$. For any set $R$ of permutations, we write $S_n(R)$ to denote the set of permutations in $S_n$ which avoid every permutation in $R$. In 1985 Simion and Schmidt showed that $|S_n(132, 213, 123)|$ is equal to the Fibonacci number $F_{n+1}$. In this paper we generalize this result in several ways. We first use a result of Mansour to show that for any permutation $\tau$ in a certain infinite family of permutations, $|S_n(132, 213, \tau)|$ is given in terms of Fibonacci numbers or $k$-generalized Fibonacci numbers. In many cases we give explicit enumerations, which we prove bijectively. We then use generating function techniques to show that for any permutation $\gamma$ in a second infinite family of permutations, $|S_n(123, 132, \gamma)|$ is also given in terms of Fibonacci numbers or $k$-generalized Fibonacci numbers. In many cases we give explicit enumerations, some of which we prove bijectively. We go on to use generating function techniques to show that for any permutation $\omega$ in a third infinite family of permutations, $|S_n(132, 2341, \omega)|$ is given in terms of Fibonacci numbers, and for any permutation $\mu$ in a fourth infinite family of permutations, $|S_n(132, 3241, \mu)|$ is given in terms of Fibonacci numbers and $k$-generalized Fibonacci numbers. In several cases we give explicit enumerations. We conclude by giving an infinite class of examples of a set $R$ of permutations for which $|S_n(R)|$ satisfies a linear homogeneous recurrence relation with constant coefficients.