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Search: id:A131905
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| A131905 |
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Those integers for which a smaller positive integer exists which has the same sum of squares of its divisors. |
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+0
2
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| 7, 26, 35, 47, 77, 91, 119, 130, 133, 141, 157, 161, 175, 182, 203, 215, 217, 249, 259, 282, 286, 287, 301, 329, 371, 385, 413, 423, 427, 434, 442, 455, 469, 471, 494, 497
(list; graph; listen)
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OFFSET
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1,1
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FORMULA
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a(n)= n-th element of {x>0: there exists a k with 0<k<x and the same sum of the squares of its divisors as x)
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EXAMPLE
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a(3)=35 because it is the third integer for which a smaller integer with the same sum of squares of its divisors exists. Divisors of 35 are 1,5,7,35 and 1+25+49+1225=1300 and the squares of the divisors of 30 are 1,4,9,25,36,100,225,900 which sum to 1300
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MATHEMATICA
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Clear[tmp]; First@Transpose[ Function[n, (If[Head[ #1] === tmp, #1 = n; Unevaluated[Sequence[]], {n, #1}] & )[tmp[DivisorSigma[2, n]]]] /@ Range[500]]
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CROSSREFS
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Cf. A069822, A131902-A131908.
Sequence in context: A012490 A063453 A098127 this_sequence A110927 A103267 A125972
Adjacent sequences: A131902 A131903 A131904 this_sequence A131906 A131907 A131908
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KEYWORD
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easy,nonn
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AUTHOR
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Peter Pein (petsie(AT)dordos.net), Jul 26 2007
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