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%I #15 Dec 20 2024 02:42:29
%S 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,
%T 27,28,29,30,21,32,33,34,33,36,37,24,28,40,20,42,43,44,45,30,33,48,49,
%U 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
%N Least k such that sigma(k) = sigma(n).
%H Antti Karttunen, <a href="/A275987/b275987.txt">Table of n, a(n) for n = 1..16384</a>
%H Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%H <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>.
%e a(11) = 6 because sigma(6) = sigma(11) = 12.
%t With[{nn = 76}, With[{s = Values@ PositionIndex@ Array[DivisorSigma[1, #] &, nn]}, Array[s[[FirstPosition[s, #][[1]], 1 ]] &, nn]]] (* _Michael De Vlieger_, Nov 16 2017 *)
%o (PARI) a(n) = {my(k = 1); while(sigma(k) != sigma(n), k++); k; }
%o (PARI) a(n) = invsigmaMin(sigma(n)); \\ _Amiram Eldar_, Dec 20 2024, using _Max Alekseyev_'s invphi.gp
%Y Cf. A000203, A069822, A286603.
%K nonn,changed
%O 1,2
%A _Altug Alkan_, Aug 15 2016