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A131723
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a(2*n) = 1-n^2, a(2*n+1) = n*(n+1).
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5
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0, 2, -3, 6, -8, 12, -15, 20, -24, 30, -35, 42, -48, 56, -63, 72, -80, 90, -99, 110, -120, 132, -143, 156, -168, 182, -195, 210, -224, 240, -255, 272, -288, 306, -323, 342, -360, 380, -399, 420, -440, 462, -483, 506, -528, 552, -575, 600, -624, 650, -675
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OFFSET
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0,2
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COMMENTS
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LINKS
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FORMULA
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G.f.: x*(2+x)/((1+x)^3*(1-x)).
a(n) = -(-1)^n*(2*n^2+8*n+3-3*(-1)^n)/8. (End)
a(n) = -2*a(n-1) + 2*a(n-3) + a(n-4) for n>3.
a(n) = -(-1)^n*floor((n+1)*(n+3)/4).
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MAPLE
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MATHEMATICA
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Table[-(-1)^n*Floor[(n + 1)*(n + 3)/4], {n, 0, 100}] (* Wesley Ivan Hurt, Jun 07 2016 *)
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PROG
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(Magma) [-(-1)^n*(2*n^2+8*n+3-3*(-1)^n)/8: n in [0..50]]; // Vincenzo Librandi, Aug 10 2011
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CROSSREFS
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KEYWORD
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sign,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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