OFFSET
0,7
LINKS
Index entries for linear recurrences with constant coefficients, signature (2, 1, 0, -6, -4, 16, -8).
FORMULA
a(n) = 2*a(n-1) + a(n-2) - 6*a(n-4) - 4*a(n-5) + 16*a(n-6) - 8*a(n-7).
G.f.: (4 x^6)/((-1 + x)^2 (-1 + 2 x^2) (-1 + 4 x^3)).
EXAMPLE
The 4 relevant subsets of {1,2,3,4,5,6} are
{1, 2, 3, 6}, {1, 2, 3, 4, 6}, {1, 2, 3, 5, 6}, and {1, 2, 3, 4, 5, 6}.
MATHEMATICA
s[n_] := s[n] = Select[Subsets[Range[n]], Length[#] >= 3 &];
a[n_] := Select[s[n], #[[1]] + #[[2]] + #[[3]] == #[[-1]] &]
Table[Length[a[n]], {n, 0, 15}]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Oct 02 2022
STATUS
approved