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Sets and subsets!

Source: Romania TST 2014 Day 1 Problem 5

January 21, 2015
combinatorics unsolvedcombinatorics

Problem Statement

Let nn be an integer greater than 11 and let SS be a finite set containing more than n+1n+1 elements.Consider the collection of all sets AA of subsets of SS satisfying the following two conditions : (a) Each member of AA contains at least nn elements of SS. (b) Each element of SS is contained in at least nn members of AA.
Determine maxAminBB\max_A \min_B |B| , as BB runs through all subsets of AA whose members cover SS , and AA runs through the above collection.