MathDB
Placing checkers in circles

Source: Belarusian TST 2024

October 27, 2024
combinatorics

Problem Statement

Given positive integers nn and knk \leq n. Consider an equilateral triangular board with side nn, which consists of circles: in the first (top) row there is one circle, in the second row there are two circles, \ldots , in the bottom row there are nn circles (see the figure below). Let us place checkers on this board so that any line parallel to a side of the triangle (there are 3n3n such lines) contains no more than kk checkers. Denote by T(k,n)T(k, n) the largest possible number of checkers in such a placement. https://i.ibb.co/bJjjK1M/Image2.jpg a) Prove that the following upper bound is true: T(k,n)k(2n+1)3T(k,n) \leq \lfloor \frac{k(2n+1)}{3} \rfloor b) Find T(1,n)T(1,n) and T(2,n)T(2,n) D. Zmiaikou