How many ways are there to arrange the numbers $1, 2, 3, .. , 15$ in some order such that for any two numbers which are $2$ or $3$ positions apart, the one on the left is greater?
Problem
Source: 2019 Dürer Math Competition Finals Day2 E12 https://artofproblemsolving.com/community/c1621835_2019_
Tags: combinatorics