Enumeration in the lattice of $q$-decreasing words

Abstract

We prove that the poset of $q$-decreasing words equipped with the componentwise order forms a lattice. We enumerate the join-irreducible elements for arbitrary $q>0$, and for any positive rational number $q$, we determine the number of coverings, intervals and meet-irreducible elements. The latter present the same structure as words over an alphabet of $2\lceil q\rceil +1$ letters avoiding $\lceil q{\rceil }^2+2\lceil q\rceil -1$ consecutive patterns of length 2. Furthermore, we analyze the asymptotic behavior of several of these quantities.

22 pages, 1 figure