Inversion polynomials for 321-avoiding permutations
  We prove a generalization of a conjecture of Dokos, Dwyer, Johnson, Sagan, and Selsor giving a recursion for the inversion polynomial of 321-avoiding permutations. We also answer a question they posed about finding a recursive formulas for the major index polynomial of 321-avoiding permutations. Other properties of these polynomials are investigated as well. Our tools include Dyck and 2-Motzkin paths, polyominoes, and continued fractions. 
Szu-En Cheng (National University of Kaohsiung), Sergi Elizalde   (Dartmouth College), Anisse Kasraoui (Universit\"at Wien),...
math.CO