Question
A collection of subsets of $\{1,2, \ldots, n\}$ has the property that each pair of subsets has at least one element in common. Prove that there are at most $2^{n-1}$ subsets in the collection.
Step 1
First, consider the case when $n = 1$. In this case, there is only one subset, the empty set, and the collection has at most $2^0 = 1$ subset, which satisfies the condition. Show more…
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