Two-sided Eulerian numbers via balls in boxes
arXiv:1209.6273
Abstract
The Eulerian numbers count permutations according to the number of descents. The two-sided Eulerian numbers count permutations according to number of descents and the number of descents in the inverse permutation. Here we derive some results for Eulerian and two-sided Eulerian numbers using an elementary "balls-in-boxes" approach. We also discuss an open conjecture of Ira Gessel about the two-sided Eulerian numbers.
Primarily expository article, to appear in Mathematics Magazine. 23 pages