OFFSET
0,3
LINKS
Index entries for linear recurrences with constant coefficients, signature (4, -5, 0, 5, -4, 1).
FORMULA
a(n) = 4*a(n-1)-5*a(n-2)+5*a(n-4)-4*a(n-5)+a(n-6).
G.f.: (8*x^2 + 14*x^3 + 8*x^4)/(1 - 4*x + 5* x^2 - 5*x^4 + 4*x^5 - x^6).
a(n) = (-1+(-1)^n-8*n^2+10*n^4)/16. [Colin Barker, Jun 10 2012]
MATHEMATICA
t = Compile[{{n, _Integer}}, Module[{s = 0},
(Do[If[w <= Abs[x - y] + Abs[y - z], s = s + 1],
{w, 1, #}, {x, 1, #}, {y, 1, #}, {z, 1, #}] &[n]; s)]];
Map[t[#] &, Range[0, 40]] (* A212673 *)
%/2 (* integers *)
LinearRecurrence[{4, -5, 0, 5, -4, 1}, {0, 0, 8, 46, 152, 378}, 40]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, May 23 2012
STATUS
approved