OFFSET
0,2
COMMENTS
For a guide to related sequences, see A211422.
LINKS
Colin Barker, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (2,0,-1,-1,0,2,-1).
FORMULA
a(n) = 2*a(n-1) - a(n-3) - a(n-4) + 2*a(n-6) - a(n-7) for n>6.
G.f.: x*(3 + 23*x + 45*x^2 + 44*x^3 + 22*x^4 + 7*x^5) / ((1 - x)^4*(1 + x)*(1 + x + x^2)). - Colin Barker, Dec 05 2017
MATHEMATICA
t = Compile[{{u, _Integer}}, Module[{s = 0}, (Do[If[w + 2 x + 3 y > 0, s = s + 1], {w, #}, {x, #}, {y, #}] &[Flatten[{Reverse[-#], #} &[Range[1, u]]]]; s)]];
Map[t[#] &, Range[0, 70]] (* A211621 *)
FindLinearRecurrence[%]
(* Peter J. C. Moses, Apr 13 2012 *)
LinearRecurrence[{2, 0, -1, -1, 0, 2, -1}, {0, 3, 29, 103, 247, 484, 843}, 36] (* Ray Chandler, Aug 02 2015 *)
PROG
(PARI) concat(0, Vec(x*(3 + 23*x + 45*x^2 + 44*x^3 + 22*x^4 + 7*x^5) / ((1 - x)^4*(1 + x)*(1 + x + x^2)) + O(x^40))) \\ Colin Barker, Dec 05 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Apr 16 2012
STATUS
approved