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A163193 Numbers n such that sigma(n) = 2*sigma(n+1). 5
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

LINKS

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

FORMULA

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

MAPLE

A163193:=proc(q) local n;

for n from 1 to q do if sigma(n)=2*sigma(n+1) then print(n); fi; od; end:

A163193 (10^10); # Paolo P. Lava, Mar 25 2013

MATHEMATICA

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

CROSSREFS

Cf. A067081, A002961, A058073.

Sequence in context: A210427 A101097 A067702 * A088832 A198311 A060930

Adjacent sequences:  A163190 A163191 A163192 * A163194 A163195 A163196

KEYWORD

nonn

AUTHOR

Vladimir Joseph Stephan Orlovsky, Jul 22 2009

EXTENSIONS

Edited by R. J. Mathar, Jul 25 2009

STATUS

approved

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Last modified August 11 08:17 EDT 2022. Contains 356055 sequences. (Running on oeis4.)