OFFSET
0,2
COMMENTS
For a guide to related sequences, see A211422.
LINKS
Index entries for linear recurrences with constant coefficients, signature (3, -3, 1).
FORMULA
a(n)= 6*A005448(n).
a(n) = 3a(n-1)-3a(n-2)+a(n-3) for n>3.
a(n) = 6-9*n+9*n^2 for n>0. G.f.: 6*x*(1+x+x^2)/(1-x)^3. [Colin Barker, Sep 09 2012]
MATHEMATICA
t = Compile[{{u, _Integer}}, Module[{s = 0}, (Do[If[-1 <= w + x + y <= 1, s = s + 1], {w, #}, {x, #}, {y, #}] &[Flatten[{Reverse[-#], #} &[Range[1, u]]]]; s)]];
Map[t[#] &, Range[0, 70]] (* A211615 *)
%/6 (* A005448 *)
FindLinearRecurrence[%]
(* Peter J. C. Moses, Apr 13 2012 *)
Join[{0}, LinearRecurrence[{3, -3, 1}, {6, 24, 60}, 40]] (* Ray Chandler, Aug 02 2015 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Apr 16 2012
STATUS
approved