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Source: Kvant Magazine No. 3 2024 M2788

May 26, 2024
combinatorics

Problem Statement

An equilateral triangle T\mathcal{T}{} with side 111 is divided by straight lines parallel to its sides into equilateral triangles with side 1. The vertices of these small triangles, except the centre of T\mathcal{T}{} are marked. Call a set of several marked points linear if[*]the marked points lie on a line \ell parallel to one of the sides of the triangle T\mathcal{T} and; [*]if two marked points on \ell are in this set, every other marked point inbetween them is in the set. How many ways are there to split all the marked points into 111 linear sets?