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Search: id:A108281
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| A108281 |
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Numbers that are both triangular and pentagonal of the second kind. |
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+0
1
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| 0, 15, 2926, 567645, 110120220, 21362755051, 4144264359690, 803965923024825, 155965244802456376, 30256453525753512135, 5869596018751378897830, 1138671371184241752666901, 220896376413724148638480980
(list; graph; listen)
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OFFSET
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1,2
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REFERENCES
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L. E. Dickson, History of the Theory of Numbers. Carnegie Institute Public. 256, Washington, DC, Vol. 1, 1919; Vol. 2, 1920; Vol. 3, 1923, see vol. 2, p. 22.
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FORMULA
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G.f.: x*(15+x)/((1-x)*(1-194*x+x^2)). a(n)=A076139(2n).
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EXAMPLE
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a(3)=567645 which is 1065(1065-1)/2 = 615(3*615+1)/2.
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PROGRAM
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(PARI) a(n)=subst((-8+15*poltchebi(2*n)-poltchebi(2*n-1))/96, x, 7)
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CROSSREFS
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Sequence in context: A110103 A126681 A126678 this_sequence A080691 A013720 A145189
Adjacent sequences: A108278 A108279 A108280 this_sequence A108282 A108283 A108284
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KEYWORD
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nonn
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AUTHOR
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Michael Somos, May 30 2005
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