A217791 Numbers k such that sigma(k) = 3*sigma(k+1).

180, 12000, 30996, 47940, 66780, 102816, 128040, 234300, 494088, 712272, 1133088, 1408212, 1623072, 1692768, 1896336, 1925196, 2024760, 2388720, 2529090, 2836008, 3423120, 3724320, 3822360, 4628760, 4750920, 7219608, 7359912, 7603488, 7749060

Offset

1,1

Links

Donovan Johnson, Table of n, a(n) for n = 1..500

Example

47940 is in the sequence because sigma(47940)=145152, sigma(47941)=48384, and 145152=3*48384.
7749060 is in the sequence because sigma(7749060)=24192000, sigma(7749061)=8064000, and 24192000=3*8064000.

Maple

A217791:=proc(q) local n;
for n from 1 to q do if sigma(n)=3*sigma(n+1) then print(n); fi; od; end:
A217791(10^10);

Mathematica

Position[Partition[DivisorSigma[1, Range[78*10^5]], 2, 1], _?(#[[1]] == 3#[[2]]&), {1}, Heads->False]//Flatten (* Harvey P. Dale, Oct 17 2016 *)

Prog

(Magma) [n: n in [1..10^7] | SumOfDivisors(n) eq 3*SumOfDivisors(n+1)]; // Bruno Berselli, Mar 25 2013

Crossrefs

Cf. A000203, A002961, A058073, A067081, A077087, A163193, A272027 (3*sigma(n)).

Sequence in context: A008378 A214818 A287022 * A035830 A244056 A091033

Adjacent sequences: A217788 A217789 A217790 * A217792 A217793 A217794

Keyword

nonn

Author

Paolo P. Lava, Mar 25 2013

Extensions

More terms from Bruno Berselli, Mar 25 2013

Status

approved