MathDB

C1

Part of 2018 JBMO Shortlist

Problems(1)

if x \in S => (x - 1) or (x+1) \in S, S has at least 4 elements

Source: JBMO Shortlist 2018 C1

7/22/2019
A set SS is called neighbouring if it has the following two properties: a) SS has exactly four elements b) for every element xx of SS, at least one of the numbers xāˆ’1x - 1 or x+1x+1 belongs to SS. Find the number of all neighbouring subsets of the set {1,2,...,n}\{1,2,... ,n\}.
combinatoricsSetsSubsets