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Search: id:A130519
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| 0, 0, 0, 0, 1, 2, 3, 4, 6, 8, 10, 12, 15, 18, 21, 24, 28, 32, 36, 40, 45, 50, 55, 60, 66, 72, 78, 84, 91, 98, 105, 112, 120, 128, 136, 144, 153, 162, 171, 180, 190, 200, 210, 220, 231, 242, 253, 264, 276, 288, 300, 312, 325, 338, 351, 364, 378, 392, 406, 420, 435, 450
(list; graph; listen)
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OFFSET
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0,6
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COMMENT
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Complementary with A130482 regarding triangular numbers, in that A130482(n)+4*a(n)=n(n+1)/2 = A000217(n).
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FORMULA
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a(n)=floor(n/4)*(n-1-2*floor(n/4))=A002265(n)*(n-1-2*A002265(n)).
a(n)=1/2*A002265(n)*(n-2+A010873(n)). G.f.: g(x)=x^4/((1-x^4)(1-x)^2) = x^4/((1+x)*(1+x^2)*(1-x)^3).
a(n) = floor((n-1)^2/8) [From Mitch Harris (maharri(AT)gmail.com), Sep 08 2008]
a(n) = A001972(n-4), n>3. - Franklin T. Adams-Watters, Jul 10 2009
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CROSSREFS
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Cf. A002264, A002266, A004526, A010872, A010873, A010874, A130481, A130483.
Sequence in context: A056168 A054041 A019293 this_sequence A001972 A005705 A139542
Adjacent sequences: A130516 A130517 A130518 this_sequence A130520 A130521 A130522
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KEYWORD
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nonn,easy
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AUTHOR
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Hieronymus Fischer (Hieronymus.Fischer(AT)gmx.de), Jun 01 2007
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EXTENSIONS
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Partially edited by R. J. Mathar, Jul 11 2009
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