On 021-Avoiding Ascent Sequences
William Y. C. Chen, Alvin Y. L. Dai, Theodore Dokos, Tim Dwyer, Bruce E. Sagan · The Electronic Journal of Combinatorics · 2013
Ascent sequences were introduced by Bousquet-Mélou, Claesson, Dukes and Kitaev in their study of $(\bf{2+2})$-free posets. An ascent sequence of length $n$ is a nonnegative integer sequence $x=x_{1}x_{2}\ldots x_{n}$ such that $x_{1}=0$ and $x_{i}\leq {\rm asc}(x_{1}x_{2}\ldots x_{i-1})+1$ for all $1