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 A143978 a(n) = floor(2*n*(n+1)/3). 8
 1, 4, 8, 13, 20, 28, 37, 48, 60, 73, 88, 104, 121, 140, 160, 181, 204, 228, 253, 280, 308, 337, 368, 400, 433, 468, 504, 541, 580, 620, 661, 704, 748, 793, 840, 888, 937, 988, 1040, 1093, 1148, 1204, 1261, 1320, 1380, 1441, 1504, 1568, 1633, 1700, 1768, 1837 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Second diagonal of array A143979, which counts certain unit squares in a lattice. First diagonal: A030511. Convolution of A042965 with A000012, convolution of A131534 with A000027, and convolution of A106510 with A000217. - L. Edson Jeffery, Jan 24 2015 LINKS Bruno Berselli, Illustration of the initial terms. Index entries for linear recurrences with constant coefficients, signature (2,-1,1,-2,1) FORMULA From R. J. Mathar, Oct 05 2009: (Start) G.f.: x*(1 + x)^2/((1 + x + x^2)*(1-x)^3). a(n) = 2*a(n-1) - a(n-2) + a(n-3) - 2*a(n-4) + a(n-5). (End) CROSSREFS Cf. A000217, A030511, A042965 (first differences), A106510, A131534, A143979. Sequence in context: A265382 A056738 A170907 * A071994 A023661 A157130 Adjacent sequences:  A143975 A143976 A143977 * A143979 A143980 A143981 KEYWORD nonn,easy AUTHOR Clark Kimberling, Sep 06 2008 STATUS approved

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Last modified November 13 10:55 EST 2018. Contains 317133 sequences. (Running on oeis4.)