A049103 Numbers k such that k and k+1 both have 6 divisors.

44, 75, 98, 116, 147, 171, 242, 243, 244, 332, 387, 507, 548, 603, 604, 724, 844, 908, 931, 963, 1075, 1083, 1251, 1324, 1412, 1467, 1556, 1587, 1675, 1772, 2523, 2524, 2636, 2644, 2763, 3283, 3356, 3411, 3508, 3788, 3987, 4075, 4203, 4204, 4418, 4491, 4804, 4868, 4923, 4924

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, entry 44, p. 103.

Links

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

Crossrefs

Intersection of A005237 and A030515.

Cf. A038400, A049104.

Programs

Mathematica

Flatten[Position[Partition[Table[DivisorSigma[0,n],{n,5000}],2,1],?(# == {6,6}&)]] (* _Harvey P. Dale, Apr 09 2012 *)

PARI

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

Formula

a(n) = A049104(n) - 1. - Zak Seidov, Feb 11 2016