OFFSET
0,3
COMMENTS
For a guide to related sequences, see A211795.
LINKS
Bo Gyu Jeong, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (3,-1,-5,5,1,-3,1).
FORMULA
a(n) = 3*a(n-1)-a(n-2)-5*a(n-3)+5*a(n-4)+a(n-5)-3*a(n-6)+a(n-7).
G.f.: 2*(1-3*x+x^2+6*x^3-3*x^4+3*x^5+5*x^6)/((1+x)^2*(1-x)^5). [Bruno Berselli, Jun 14 2012]
a(n) = (2*(n-3)*n*(5*n^2-15*n+31)-3*(2*n-3)*(-1)^n+87)/48. [Bruno Berselli, Jun 14 2012]
MATHEMATICA
t = Compile[{{n, _Integer}}, Module[{s = 0},
(Do[If[Abs[w - x] > Abs[x - y] + Abs[y - z], s = s + 1],
{w, 1, #}, {x, 1, #}, {y, 1, #}, {z, 1, #}] &[n]; s)]];
Map[t[#] &, Range[0, 40]] (* A212573 *)
%/2 (* integers *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, May 22 2012
STATUS
approved