A320514 Numbers k such that sigma(sigma(k)) < 2*k + 1.
1, 2, 4, 9, 13, 16, 25, 37, 43, 49, 61, 64, 67, 73, 81, 97, 109, 121, 151, 157, 163, 169, 181, 193, 211, 225, 229, 241, 277, 283, 289, 313, 331, 337, 361, 373, 397, 409, 421, 433, 457, 487, 523, 529, 541, 547, 577, 601, 613, 625, 631, 661, 673, 691, 709, 729, 733, 751, 757, 787, 823, 829, 841, 853, 877
Offset: 1
Keywords
Links
- Amiram Eldar, Table of n, a(n) for n = 1..10000
Programs
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Maple
isA320514 := n -> sigma(sigma(n)) < 2*n+1: select(isA320514, [$1..1000]);
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Mathematica
Select[Range[1000], DivisorSigma[1, DivisorSigma[1, #]] < 2*# + 1 &] (* Vaclav Kotesovec, Oct 14 2018 *)
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PARI
isok(n) = sigma(sigma(n)) < 2*n + 1; \\ Michel Marcus, Oct 14 2018