A067081 Numbers k such that sigma(k+1) = 2*sigma(k).

5, 125, 1253, 1673, 3127, 5191, 7615, 12035, 43817, 47795, 48559, 49955, 56975, 58373, 61721, 63545, 68033, 78395, 97411, 106001, 108473, 109939, 111451, 111683, 123497, 123797, 131659, 137081, 192065, 192279, 224561, 228377, 274273, 280313, 290417, 290719, 351815

Offset: 1

Author

Benoit Cloitre, Feb 18 2002

Keywords

• nonn

Comments

Sequence seems to consist only of odd values.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..2500 (terms 1..761 from Harry J. Smith)

Crossrefs

Cf. A000203 (sigma).

Programs

Mathematica

f[n_]:=DivisorSigma[1,n]; lst={};Do[If[2*f[n]==f[n+1],AppendTo[lst,n]],{n,9!}];lst (* Vladimir Joseph Stephan Orlovsky, Jul 22 2009 *)
Position[Partition[DivisorSigma[1,Range[300000]],2,1],?(2#[[1]]==#[[2]]&),1,Heads->False]//Flatten (* _Harvey P. Dale, Oct 21 2024 *)

PARI

isok(k) = { sigma(k+1) == 2*sigma(k) } \\ Harry J. Smith, May 08 2010

Extensions

More terms from Harry J. Smith, May 08 2010