A212981 - OEIS

A212981

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

0, 2, 8, 21, 43, 77, 125, 190, 274, 380, 510, 667, 853, 1071, 1323, 1612, 1940, 2310, 2724, 3185, 3695, 4257, 4873, 5546, 6278, 7072, 7930, 8855, 9849, 10915, 12055, 13272, 14568, 15946, 17408, 18957, 20595, 22325, 24149, 26070, 28090

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OFFSET

0,2

COMMENTS

For a guide to related sequences, see A212959.

LINKS

Table of n, a(n) for n=0..40.

Index entries for linear recurrences with constant coefficients, signature (3,-2,-2,3,-1).

FORMULA

a(n) = 3*a(n-1)-2*a(n-2)-2*a(n-3)+3*a(n-4)-a(n-5).

G.f.: f(x)/g(x), where f(x)=2*x + 2*x^2 + x^3 and g(x)=(1+x)*(1-x)^4.

a(n) = (20*n^3+42*n^2+28*n+3*(1-(-1)^n))/48. - Luce ETIENNE, Feb 17 2015

MATHEMATICA

t = Compile[{{n, _Integer}}, Module[{s = 0},

(Do[If[w <= x + y && x < y, s = s + 1],

{w, 0, n}, {x, 0, n}, {y, 0, n}]; s)]];

m = Map[t[#] &, Range[0, 60]] (* A212981 *)

LinearRecurrence[{3, -2, -2, 3, -1}, {0, 2, 8, 21, 43}, 50] (* Harvey P. Dale, Jul 31 2013 *)

CROSSREFS

Cf. A212959.

Sequence in context: A358681 A090612 A355760 * A051744 A062443 A275740

Adjacent sequences: A212978 A212979 A212980 * A212982 A212983 A212984

KEYWORD

nonn,easy

AUTHOR

Clark Kimberling, Jun 04 2012

STATUS

approved