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 A387405 #14 Aug 31 2025 10:48:27
%S A387405 18,54,196,1521,486,1372,15376,24025,4374,1032256,67228,39366,476656,
%T A387405 257049,744775,27237961,354294,3341637,3294172,23088025,870899121,
%U A387405 131096512,3188646,4990433449,18837288001,28697814,43647148561,458066416,161414428,131785698529,8362054027,274810802176,564736653,508339054441,258280326
%N A387405 a(n) is the least k that is a multiple of d=A240991(n), with abundancy ratio sigma(k)/k equal to (sigma(d)+1)/d), or -1 if no such k exists.
%H A387405 Antti Karttunen, <a href="/A387405/b387405.txt">Table of n, a(n) for n = 1..67</a> (based on the b-file of A240991 as computed by Michel Marcus and Giovanni Resta)
%H A387405 C. A. Holdener and J. A. Holdener, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL23/Holdener/holdener4.html">Characterizing Quasi-Friendly Divisors</a>, Journal of Integer Sequences, Vol. 23 (2020), Article 20.8.4.
%F A387405 a(n) = A253560(A240991(n)) = A006530(A240991(n)) * A240991(n). - Conjectured, most likely true, especially if A240991 is a subsequence of A387406.
%o A387405 (PARI) A387405(n) = { my(d=A240991(n), r=(sigma(d)+1)/d); forstep(k=d,oo,d,if(sigma(k)/k==r, return(k))); }
%Y A387405 Cf. A006530, A240991, A253560, A387406.
%K A387405 nonn,new
%O A387405 1,1
%A A387405 _Antti Karttunen_, Aug 30 2025