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Search: id:A134481
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| 1, 5, 20, 50, 100, 175, 280, 420, 600, 825, 1100, 1430, 1820, 2275
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OFFSET
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0,2
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COMMENT
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If Y is a 5-subset of an n-set X then, for n>=8, a(n-7) is the number of 4-subsets of X having exactly one element in common with Y. - Milan R. Janjic (agnus(AT)blic.net), Dec 28 2007
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FORMULA
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Binomial transform of [1, 4, 11, 4, 1, -1, 1, -1, 1, -1,...]. Also, 1 followed by 5 * A000292, the tetrahedral numbers; i.e. 1, then 5 * (1, 4, 10, 25, 35,...).
a(n)=5*binomial(n-5,3), n=8,9,...,20. - Milan R. Janjic (agnus(AT)blic.net), Dec 28 2007
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EXAMPLE
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a(2) = 20 = sum of row 3 terms of triangle A134480: (9 + 7 + 4).
a(3) = 50 = (1, 3, 3, 1) dot (1, 4, 11, 4) = (1 + 12 + 33 + 4).
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CROSSREFS
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Cf. A134480, A000292.
Adjacent sequences: A134478 A134479 A134480 this_sequence A134482 A134483 A134484
Sequence in context: A147002 A005287 A147488 this_sequence A062158 A034133 A006504
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KEYWORD
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nonn
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AUTHOR
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Gary W. Adamson (qntmpkt(AT)yahoo.com), Oct 27 2007
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