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 A011865 a(n) = floor( n*(n - 1)/12 ). 7
 0, 0, 0, 0, 1, 1, 2, 3, 4, 6, 7, 9, 11, 13, 15, 17, 20, 22, 25, 28, 31, 35, 38, 42, 46, 50, 54, 58, 63, 67, 72, 77, 82, 88, 93, 99, 105, 111, 117, 123, 130, 136, 143, 150, 157, 165, 172, 180, 188, 196, 204, 212, 221, 229, 238, 247, 256, 266, 275, 285, 295, 305, 315, 325, 336 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,7 LINKS FORMULA From R. J. Mathar, Apr 15 2010: (Start) a(n) = +2*a(n-1) -a(n-2) +a(n-3) -2*a(n-4) +a(n-5) -a(n-6) +2*a(n-7) -a(n-8) +a(n-9) -2*a(n-10) +a(n-11). G.f.: x^4*(x^4+1-x^3-x+x^2) / ((1-x)^3*(1+x+x^2)*(x^2+1)*(x^4-x^2+1)). (End) MAPLE seq(floor(binomial(n-1, 2)/6), n=1..53); # Zerinvary Lajos, Jan 12 2009 CROSSREFS Sequence in context: A225086 A062413 A064313 * A085680 A253186 A321195 Adjacent sequences:  A011862 A011863 A011864 * A011866 A011867 A011868 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified April 24 12:21 EDT 2019. Contains 322429 sequences. (Running on oeis4.)