Let n and k be given natural numbers, and let A be a set such that ∣A∣≤k+1n(n+1). For i=1,2,...,n+1, let Ai be sets of size n such that ∣Ai∩Aj∣≤k(i=j) , A=i=1⋃n+1Ai. Determine the cardinality of A.
K. Corradi combinatorics proposedcombinatorics