MathDB
Putnam 1956 A5

Source: Putnam 1956

July 5, 2022
Putnambinomial coefficientscombinatorics

Problem Statement

Call a subset of {1,2,…,n}\{1,2,\ldots, n\} unfriendly if no two of its elements are consecutive. Show that the number of unfriendly subsets with kk elements is (n−k+1k).\binom{n-k+1}{k}.
Back to ProblemsView on AoPS