OFFSET
0,7
LINKS
Index entries for linear recurrences with constant coefficients, signature (2,1,-1,-2,-1,2).
FORMULA
a(n) = 2*a(n-1) + a(n-2) - a(n-3) - 2*a(n-4) - a(n-5) + 2*a(n-6).
G.f.: -(x^5/((-1 + x)^2 (1 + x) (-1 + 2 x) (1 + x + x^2))).
EXAMPLE
The 2 relevant subsets of {1,2,3,4,5,6} are {1, 2, 5} and {1,2,3,6}.
MATHEMATICA
s[n_] := s[n] = Select[Subsets[Range[n]], Length[#] >= 3 &];
a[n_] := Select[s[n], #[[1]] + #[[2]] == #[[-1]] - #[[-2]] &]
Table[Length[a[n]], {n, 0, 16}]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Oct 02 2022
STATUS
approved