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A problem on Sets

Source: Federal Mathematical Competition of Serbia and Montenegro 2004

May 14, 2018
Setscombinatorics

Problem Statement

Let A={1,2,3,...,11}A = \{1,2,3, . . . ,11\}. How many subsets BB of AA are there, such that for each nāˆˆ{1,2,...,8}n\in \{1,2, . . . ,8\}, if nn and n+2n+2 are in BB then at least one of the numbers n+1 n+1 and n+3n+3 is also in BB?
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