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Coloring natural numbers

Source: Iran 3rd round 2012-Combinatorics exam-P2

September 20, 2012
combinatorics proposedcombinatorics

Problem Statement

Suppose s,k,t∈Ns,k,t\in \mathbb N. We've colored each natural number with one of the kk colors, such that each color is used infinitely many times. We want to choose a subset A\mathcal A of N\mathbb N such that it has tt disjoint monochromatic ss-element subsets. What is the minimum number of elements of AA?
Proposed by Navid Adham