Inversion sequences avoiding the pattern 010

Abstract

Inversion sequences are integer sequences $(\sigma_1, \dots, \sigma_n)$ such that $0 \leqslant \sigma_i < i$ for all $1 \leqslant i \leqslant n$. The study of pattern-avoiding inversion sequences began in two independent articles by Mansour-Shattuck and Corteel-Martinez-Savage-Weselcouch in 2015 and 2016. These two initial articles solved the enumeration of inversion sequences avoiding a single pattern for every pattern of length 3 except the patterns 010 and 100. The case 100 was recently solved by Mansour and Yildirim. We solve the final case by making use of a decomposition of inversion sequences avoiding the pattern 010. Our decomposition needs to take into account the maximal value, and the number of distinct values occurring in the inversion sequence. We then expand our method to solve the enumeration of inversion sequences avoiding the pairs of patterns $\{010, 000\}, \{010, 110\}, \{010, 120\}$, and the Wilf-equivalent pairs $\{010, 201\} \sim \{010, 210\}$. For each family of pattern-avoiding inversion sequences considered, its enumeration requires the enumeration of some family of constrained words avoiding the same patterns, a question which we also solve.