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 A329821 #17 Dec 26 2024 03:52:07
%S A329821 1,2,3,5,4,7,11,9,13,8,17,19,23,12,29,25,31,22,37,18,27,41,43,47,53,
%T A329821 39,49,59,61,32,67,71,73,45,79,83,89,36,50,77,97,101,103,107,109,91,
%U A329821 113,95,81,75,82,64,127,131,121,137,139,119,149,151,125
%N A329821 Largest k for which sigma(k) = A002191(n), where A002191 = range of sigma, the sum-of-divisors function A000203.
%H A329821 Amiram Eldar, <a href="/A329821/b329821.txt">Table of n, a(n) for n = 1..10000</a>
%H A329821 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%F A329821 a(n) = A085790(m,A054973(m)) with m = A002191(n).
%e A329821 The possible values of sigma(x) are A002191 = {1, 3, 4, 6, 7, 8, 12, ...}.
%e A329821 The 7th value is 12 = sigma(x) for x = 6 or 11. Since 11 is the largest such value, a(7) = 11.
%o A329821 (PARI) A329821(n)=vecmax(invsigma(A002191(n))) \\ see Alekseyev link for invsigma(). An invsigmaMax() function is announced.
%Y A329821 Cf. A000203 (sigma), A002191 (range of sigma), A085790 (table of pre-images of x in A002191), A054973 (number of solutions of sigma(x) = n).
%Y A329821 Cf. A051444 (smallest k such that sigma(k) = n).
%K A329821 nonn
%O A329821 1,2
%A A329821 _M. F. Hasler_, Nov 22 2019