crossing ordered quadruple of corners in a regular 2013-gon
Source: Norwegian Mathematical Olympiad 2013 - Abel Competition p4a
September 4, 2019
combinatorics
Problem Statement
An ordered quadruple of corners in a regular -gon is called crossing if the four corners are all different, and the line segment from to intersects the line segment from to . How many crossing quadruples are there in the -gon?