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A320515
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Squarefree k > 1 with sigma(sigma(k)) < 2*k + 1.
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2
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2, 13, 37, 43, 61, 67, 73, 97, 109, 151, 157, 163, 181, 193, 211, 229, 241, 277, 283, 313, 331, 337, 373, 397, 409, 421, 433, 457, 487, 523, 541, 547, 577, 601, 613, 631, 661, 673, 691, 709, 733, 751, 757, 787, 823, 829, 853, 877, 883, 907, 937, 997
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OFFSET
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1,1
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COMMENTS
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Conjecturally a subsequence of A085497.
This conjecture is false, the first counterexample is a(113) = 2257 = 37 * 61 which is the least composite term in this sequence. - Amiram Eldar, Jun 17 2020
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LINKS
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MAPLE
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isA320515 := n -> (n > 1) and issqrfree(n) and (sigma(sigma(n)) < 2*n+1):
select(isA320515, [$1..1000]);
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MATHEMATICA
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Rest[Select[Range[1000], SquareFreeQ[#] && DivisorSigma[1, DivisorSigma[1, #]] < 2*# + 1 &]] (* Vaclav Kotesovec, Oct 14 2018 *)
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PROG
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(PARI) isok(n) = (n>1) && issquarefree(n) && (sigma(sigma(n)) < 2*n + 1); \\ Michel Marcus, Oct 14 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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