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Search: id:A138613
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| A138613 |
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a(n) = sigma_2(a(n-1)): sum of squares of divisors of a(n-1) with a(1)=2. |
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+0
3
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| 2, 5, 26, 850, 943950, 1318281510000, 2755017380686402917800000, 10751241448828145947377163154256928532205365443200, 18665349732071130524849313082094840330754079952842290235019994667394396474887720\ 1760000000000000000
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Are all following terms divisible by 10?
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LINKS
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Robert Gerbicz, Table of n, a(n) for n = 1..12
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FORMULA
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a(n>=1) = sigma_2(a(n-1)) = A001157(a(n-1)), a(1)=2.
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EXAMPLE
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a(1)=2, divisors of 2: {1,2}, hence a(2)=1^2+2^2=5;
a(2)=5, divisors of 5: {1,5}, hence a(3)=1^2+5^2=26;
a(3)=26, divisors of 5: {1,2,13,26}, hence a(4)=1^2+2^2+13^2+26^2=850.
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MATHEMATICA
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NestList[DivisorSigma[2, # ]&, 2, 9]
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CROSSREFS
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Cf. A001157, A033766, A079488.
Sequence in context: A064006 A003095 A023362 this_sequence A090744 A041571 A042259
Adjacent sequences: A138610 A138611 A138612 this_sequence A138614 A138615 A138616
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KEYWORD
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nonn
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AUTHOR
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Zak Seidov (zakseidov(AT)yahoo.com), May 14 2008
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