• 1 University of Haifa [Haifa]
• 2 Bilkent University [Ankara]

We study the enumeration of inversion sequences that avoid the pattern 021 and another pattern of length four. We determine the generating trees for all possible pattern pairs and compute the corresponding generating functions. We introduce the concept of dregular generating trees and conjecture that for any 021-avoiding pattern τ , the generating tree T ({021, τ }) is d-regular for some integer d.

• 05A15 - Exact enumeration problems, generating functions
• 05C05 - Trees
• 11B83 - Special sequences and polynomials