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Search: id:A131903
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| A131903 |
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Integers for which a smaller positive integer exists which has the same number of divisors. |
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+0
3
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| 3, 5, 7, 8, 9, 10, 11, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 61, 62, 63
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Appears to be the complement of A007416. - T. D. Noe (noe(AT)sspectra.com), Jul 26 2007
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FORMULA
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a(n) = n-th element of the set {x>0 : there exists a k with 0<k<x and the same number of divisors as x}.
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EXAMPLE
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a(4)=8 because it is the fourth integer for which a smaller integer with the same number of divisors exists (after 3, 5 and 7). Divisors of 8 are 1,2,4,8 which are four and the divisors of 6 which is less than 8 are (1, 2, 3, 6) which are four.
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MATHEMATICA
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Clear[tmp]; Function[n, If[Head[ #1] === tmp, #1 = n; Unevaluated[Sequence[]], n] & [tmp[DivisorSigma[0, n]]]] /@ Range[64]
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CROSSREFS
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Cf. A069822, A131902-A131908.
Sequence in context: A008520 A100318 A167707 this_sequence A141114 A136443 A020491
Adjacent sequences: A131900 A131901 A131902 this_sequence A131904 A131905 A131906
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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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