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 A328416 #8 Oct 18 2019 17:06:48 %S A328416 7,4,1,5,2,10,21,6,42,90,150,30,78,210,2730,690,1050 %N A328416 Smallest k such that (Z/mZ)* = C_2 X C_(2k) has exactly n solutions for m, or 0 if no such k exists, where (Z/mZ)* is the multiplicative group of integers modulo m. %C A328416 Conjecture: a(n) > 0 for all n. That is to say, every number occurs in A328412. %C A328416 It seems that most terms are congruent to 2 modulo 4. %H A328416 Wikipedia, <a href="https://en.wikipedia.org/wiki/Multiplicative_group_of_integers_modulo_n">Multiplicative group of integers modulo n</a> %e A328416 (Z/mZ)* = C_2 X C_42 has exactly 6 solutions m = 129, 147, 172, 196, 258, 294; for any k < 21, (Z/mZ)* = C_2 X C_(2k) has either fewer than or more than 6 solutions, so a(6) = 21. %o A328416 (PARI) a(n) = my(k=1); while(A328412(k)!=n, k++); k \\ See A328412 for its program %Y A328416 Cf. A328412. %K A328416 nonn,hard,more %O A328416 0,1 %A A328416 _Jianing Song_, Oct 14 2019