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A212981 Number of (w,x,y) with all terms in {0,...,n} and w <= x + y and x < y.

%I A212981 #20 Jun 13 2015 00:54:15
%S A212981 0,2,8,21,43,77,125,190,274,380,510,667,853,1071,1323,1612,1940,2310,
%T A212981 2724,3185,3695,4257,4873,5546,6278,7072,7930,8855,9849,10915,12055,
%U A212981 13272,14568,15946,17408,18957,20595,22325,24149,26070,28090
%N A212981 Number of (w,x,y) with all terms in {0,...,n} and w <= x + y and x < y.
%C A212981 For a guide to related sequences, see A212959.
%H A212981 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (3,-2,-2,3,-1).
%F A212981 a(n) = 3*a(n-1)-2*a(n-2)-2*a(n-3)+3*a(n-4)-a(n-5).
%F A212981 G.f.: f(x)/g(x), where f(x)=2*x + 2*x^2 + x^3 and g(x)=(1+x)*(1-x)^4.
%F A212981 a(n) = (20*n^3+42*n^2+28*n+3*(1-(-1)^n))/48. - _Luce ETIENNE_, Feb 17 2015
%t A212981 t = Compile[{{n, _Integer}}, Module[{s = 0},
%t A212981 (Do[If[w <= x + y && x < y, s = s + 1],
%t A212981 {w, 0, n}, {x, 0, n}, {y, 0, n}]; s)]];
%t A212981 m = Map[t[#] &, Range[0, 60]]   (* A212981 *)
%t A212981 LinearRecurrence[{3,-2,-2,3,-1},{0,2,8,21,43},50] (* _Harvey P. Dale_, Jul 31 2013 *)
%Y A212981 Cf. A212959.
%K A212981 nonn,easy
%O A212981 0,2
%A A212981 _Clark Kimberling_, Jun 04 2012