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

1,1

Comments

Sequence seems to consist only of odd values.

Links

Harry J. Smith, Table of n, a(n) for n = 1..761

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 *)

Prog

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

Crossrefs

Sequence in context: A193612 A195949 A354654 * A195801 A154022 A013710

Adjacent sequences: A067078 A067079 A067080 * A067082 A067083 A067084

Keyword

nonn

Author

Benoit Cloitre, Feb 18 2002

Extensions

Terms added by Harry J. Smith, May 08 2010

Status

approved