# Jay Pantone

## Pattern Avoidance in Forests of Binary Shrubs

with David Bevan, Derek Levin, Peter Nugent, Lara Pudwell, Manda Riehl and ML Tlachac

We investigate pattern avoidance in permutations satisfying some additional restrictions. These are naturally considered in terms of avoiding patterns in linear extensions of certain forest-like partially ordered sets, which we call binary shrub forests. In this context, we enumerate forests avoiding patterns of length three. In four of the five non-equivalent cases, we present explicit enumerations by exhibiting bijections with certain lattice paths bounded above by the line $$y=\ell x$$, for some $$\ell\in\mathbb{Q}^+$$, one of these being the celebrated Duchon's club paths with $$\ell=2/3$$. In the remaining case, we use the machinery of analytic combinatorics to determine the minimal polynomial of its generating function, and deduce its growth rate.

• A210277 - the number of binary shrubs with $$3n$$ nodes
• A001764 - the number of binary shrubs with $$3n$$ nodes avoiding $$123$$
• A002293 - the number of binary shrubs with $$3n$$ nodes avoiding $$132$$
• A144097 - the number of binary shrubs with $$3n$$ nodes avoiding $$213$$ (also avoiding $$312$$)
• A060941 - the number of binary shrubs with $$3n$$ nodes avoiding $$231$$
• A257995 - the number of binary shrubs with $$3n$$ nodes avoiding $$321$$