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A131179
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a(n) = if n mod 2 == 0 then n*(n+1)/2, otherwise (n-1)*n/2 + 1.
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3
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0, 1, 3, 4, 10, 11, 21, 22, 36, 37, 55, 56, 78, 79, 105, 106, 136, 137, 171, 172, 210, 211, 253, 254, 300, 301, 351, 352, 406, 407, 465, 466, 528, 529, 595, 596, 666, 667, 741, 742, 820, 821, 903, 904, 990, 991, 1081, 1082, 1176, 1177, 1275, 1276, 1378, 1379, 1485, 1486
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OFFSET
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0,3
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LINKS
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FORMULA
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G.f.: -x*(1+2*x-x^2+2*x^3) / ( (1+x)^2*(x-1)^3 ). - R. J. Mathar, Sep 05 2012
a(n) = ( n^2+1+(n-1)*(-1)^n )/2. - Luce ETIENNE, Aug 19 2014
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MATHEMATICA
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LinearRecurrence[{1, 2, -2, -1, 1}, {0, 1, 3, 4, 10}, 60] (* Jean-François Alcover, Feb 12 2016 *)
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PROG
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(Haskell)
a131179 n = (n + 1 - m) * n' + m where (n', m) = divMod n 2
(Python)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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Philippe LALLOUET, Sep 16 2007
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STATUS
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approved
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