A049104 Numbers k such that k and k-1 both have 6 divisors.

45, 76, 99, 117, 148, 172, 243, 244, 245, 333, 388, 508, 549, 604, 605, 725, 845, 909, 932, 964, 1076, 1084, 1252, 1325, 1413, 1468, 1557, 1588, 1676, 1773, 2524, 2525, 2637, 2645, 2764, 3284, 3357, 3412, 3509, 3789, 3988, 4076, 4204, 4205, 4419, 4492, 4805, 4869, 4924, 4925

Offset: 1

Author

N. J. A. Sloane, Felice Russo, Olivier Gérard

Keywords

• nonn

References

• David Wells, The Penguin Dictionary of Curious and Interesting Numbers, Penguin Books, 1986, p. 103.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Vincenzo Librandi)

Crossrefs

Cf. A038400, A049103.

Programs

Mathematica

Flatten[Position[Partition[Table[DivisorSigma[0, n-1], {n, 5000}], 2, 1], ?(#=={6, 6} &)]] (* _Vincenzo Librandi, Oct 21 2012 *)

PARI

isok(n) = (numdiv(n) == 6) && (numdiv(n-1) == 6); \\ Michel Marcus, Feb 11 2016

Formula

a(n) = A049103(n)+1. - Zak Seidov, Feb 11 2016