A067130 Numbers k such that sigma(k-1) divides sigma(k+1).

2, 34, 55, 119, 285, 367, 835, 849, 919, 1241, 1505, 1559, 2911, 2914, 2939, 3305, 4149, 4188, 6111, 6903, 7170, 7913, 9360, 10251, 10541, 12566, 15086, 17273, 17759, 17815, 19005, 19689, 19919, 21411, 21462, 24882, 25020, 26610, 28125, 30593, 30789, 31485

Offset: 1

Author

Benoit Cloitre, Feb 18 2002

Keywords

• nonn

Links

• Donovan Johnson, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000203 (sigma), A055574.

Programs

Mathematica

Flatten[Position[Partition[DivisorSigma[1,Range[32000]],3,1],?( Divisible[ Last[#],First[#]]&),{1},Heads->False]]+1 (* _Harvey P. Dale, Jan 13 2015 *)

PARI

isok(n) = sigma(n+1) % sigma(n-1) == 0; \\ Michel Marcus, Nov 20 2013

Extensions

More terms from Michel Marcus, Nov 20 2013