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Search: id:A089207
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| 6, 40, 126, 288, 550, 936, 1470, 2176, 3078, 4200, 5566, 7200, 9126, 11368, 13950, 16896, 20230, 23976, 28158, 32800, 37926, 43560, 49726, 56448, 63750, 71656, 80190, 89376, 99238, 109800, 121086, 133120, 145926, 159528, 173950, 189216
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Yet another parametric representation of the solutions of the Diophantine equation x^2 - y^2 = z^3 is (3n^3, n^3, 2n^2). By taking the sum x+y+z we get a(n) = 4n^3 + 2n^2.
If Y is a 3-subset of an 2n-set X then, for n>=5, a(n-2) is the number of 5-subsets of X having at least two elements in common with Y. - Milan R. Janjic (agnus(AT)blic.net), Dec 16 2007
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FORMULA
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a(n)=2*A099721(n) = 4*a(n-1)-6*a(n-2)+4*a(n-3)-a(n-4). G.f.: 2*x*(3+8*x+x^2)/(x-1)^4. [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Apr 20 2009]
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CROSSREFS
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Cf. A085409, A087887.
Sequence in context: A110424 A114079 A002595 this_sequence A027777 A073773 A001919
Adjacent sequences: A089204 A089205 A089206 this_sequence A089208 A089209 A089210
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KEYWORD
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nonn,easy
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AUTHOR
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Jun Mizuki (suzuki32(AT)sanken.osaka-u.ac.jp), Dec 09 2003
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EXTENSIONS
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More terms from Ray Chandler (rayjchandler(AT)sbcglobal.net), Feb 15 2004
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