A275987 Least k such that sigma(k) = sigma(n).

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 6, 12, 13, 14, 14, 16, 10, 18, 19, 20, 21, 22, 14, 24, 16, 20, 27, 28, 29, 30, 21, 32, 33, 34, 33, 36, 37, 24, 28, 40, 20, 42, 43, 44, 45, 30, 33, 48, 49, 50, 30, 52, 34, 54, 30, 54, 57, 40, 24, 60, 61, 42, 63, 64, 44, 66, 67, 68, 42, 66, 30, 72, 73, 74, 48

Offset: 1

Author

Altug Alkan, Aug 15 2016

Keywords

• nonn

Examples

			a(11) = 6 because sigma(6) = sigma(11) = 12.
		

Links

• Antti Karttunen, Table of n, a(n) for n = 1..16384
• Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).
• Index entries for sequences related to sigma(n).

Crossrefs

Cf. A000203, A069822, A286603.

Programs

Mathematica

With[{nn = 76}, With[{s = Values@ PositionIndex@ Array[DivisorSigma[1, #] &, nn]}, Array[s[[FirstPosition[s, #][[1]], 1 ]] &, nn]]] (* Michael De Vlieger, Nov 16 2017 *)

PARI

a(n) = {my(k = 1); while(sigma(k) != sigma(n), k++); k; }

PARI

a(n) = invsigmaMin(sigma(n)); \\ Amiram Eldar, Dec 20 2024, using Max Alekseyev's invphi.gp