A163193 Numbers k such that sigma(k) = 2*sigma(k+1).

12, 70, 88, 204, 220, 1750, 1888, 2958, 8142, 8632, 9114, 14664, 18414, 18762, 20118, 20712, 25194, 45520, 64206, 65652, 65964, 77814, 79338, 79824, 85096, 90804, 103410, 103644, 117822, 158946, 163938, 176364, 185776, 186612, 194416, 202656, 203898, 245632

Offset: 1

Author

Vladimir Joseph Stephan Orlovsky, Jul 22 2009

Keywords

• nonn

Comments

The cases sigma(k) = 3*sigma(k+1) are rarer: k=180, 12000, 30996, 47940, ... [R. J. Mathar, Jul 25 2009]

Links

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

Crossrefs

Cf. A067081, A002961, A058073.

Programs

Mathematica

f[n_]:=DivisorSigma[1,n]; lst={};Do[If[f[n]==f[n+1]*2,AppendTo[lst,n]], {n,9!}];lst

Formula

{k: A000203(k) = 2*A000203(k+1)}.

Extensions

Edited by R. J. Mathar, Jul 25 2009