This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A173430 #13 Jul 22 2025 07:50:31
%S A173430 1,15,15,15,6,6,15,15,14,10,12,12,14,14,15,104,18,18,20,20,104,22,24,
%T A173430 24,104,26,40,28,30,30,104,104,33,34,48,91,38,38,56,40,42,42,44,44,45,
%U A173430 46,48,48,80,255,51,52,54,54,72,56,80,58,60,60,62,62,104,255,84,66,68,68
%N A173430 Last of consecutive coprime iterations of sum-of-divisors function.
%D A173430 Oystein Ore, Number Theory and Its History, 1988, Dover Publications, ISBN 0486656209, pp. 88-96.
%H A173430 Amiram Eldar, <a href="/A173430/b173430.txt">Table of n, a(n) for n = 1..10000</a>
%H A173430 Graeme L. Cohen and Herman J. J. te Riele, <a href="https://doi.org/10.1080/10586458.1996.10504580">Iterating the sum-of-divisors function</a>, Experimental Mathematics, Vol. 5, No. 2 (1996), pp. 91-100.
%H A173430 Leonard Eugene Dickson, History of the Theory of Numbers, <a href="http://books.google.com/books?id=mbk9AAAAYAAJ">Volume I</a>, Divisibility and Primality, Carnegie Institution of Washington, 1919, Chapters II and X
%e A173430 Calculating sum-of-divisors ( ... sum-of-divisors ( sum-of-divisors ( 4 ) ) ... ) the iterates are 4, 7, 8, 15, 24, ... .
%e A173430 The initial, consecutive, pairwise, coprime iterates are 4, 7, 8, 15, so a(4) = 15 .
%e A173430 Here sigma ( 4 ) = 7, sigma ( sigma ( 4 ) ) = sigma ( 7 ) = 8, etc.
%t A173430 a[1] = 1; a[n_] := Module[{k = n}, While[CoprimeQ[k, (s = DivisorSigma[1, k])], k = s]; k]; Array[a, 68] (* _Amiram Eldar_, Sep 02 2019 *)
%Y A173430 Cf. A129246 and the references there, A019294, A019295, A000203, A051027, A019284, A019277.
%K A173430 easy,nonn
%O A173430 1,2
%A A173430 _Walter Nissen_, Feb 18 2010