MathDB
Set of positive integers and coloring points in red

Source: 2023 Girls in Mathematics Tournament- Level B, Problem 3

October 29, 2023
combinatorics

Problem Statement

Let SS be a set not empty of positive integers and ABAB a segment with, initially, only points AA and BB colored by red. An operation consists of choosing two distinct points X,YX, Y colored already by red and nSn\in S an integer, and painting in red the nn points A1,A2,...,AnA_1, A_2,..., A_n of segment XYXY such that XA1=A1A2=A2A3=...=An1An=AnYXA_1= A_1A_2= A_2A_3=...= A_{n-1}A_n= A_nY and XA1<XA2<...<XAnXA_1<XA_2<...<XA_n. Find the least positive integer mm such exists a subset SS of {1,2,..,m}\{1,2,.., m\} such that, after a finite number of operations, we can paint in red the point KK in the segment ABAB defined by AKKB=27092022\frac{AK}{KB}= \frac{2709}{2022}. Also, find the number of such subsets for such a value of mm.