consider a subset of the first N positive integers:
S ⊆ {1, ..., N}

call S "mid" if, for every integer n, if n-1 and n+1 are elements of S, then n is in S.

#MathSky 🧮 #iTeachMath ♾️

example: if N=5, the set {2,3,4,5} is a mid subset.

⭐️ find a formula f(N) for the # of mid subsets of {1, ..., N} ⭐️