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 A306448 #16 Mar 06 2021 06:00:30
%S A306448 4,8,28,52,121,364,532,616,1036,1288,3052,3751,4376,4636,4961,5356,
%T A306448 6364,7381,8744,11011,11476,12124,15964,19096,19684,21196,21736,24388,
%U A306448 26596,29161,31876,32791,37576,40132,45676,47972,53092,61831,67276,72136,80476,80956,86296
%N A306448 Pseudoprimes to base 9 that are not squarefree.
%C A306448 Numbers k that are not squarefree and satisfy 9^(k-1) == 1 (mod k).
%C A306448 Any term is divisible by the square of a base-9 Wieferich prime ({2} U {base-3 Wieferich primes} = {2} U A014127 = {2, 11, 1006003, ...}).
%C A306448 Intersection of A020138 and A013929.
%H A306448 Amiram Eldar, <a href="/A306448/b306448.txt">Table of n, a(n) for n = 1..10000</a>
%o A306448 (PARI) for(n=1, 10^5, if(Mod(9, n)^(n-1)==1 && !issquarefree(n), print1(n, ", ")))
%Y A306448 Pseudoprimes to base b that are not squarefree: A158358 (b=2), A244065 (b=3), A243010 (b=5), A243089 (b=7), A243090 (b=8), this sequence (b=9), A306449 (b=10).
%Y A306448 Cf. also A014127, A020138, A013929.
%K A306448 nonn
%O A306448 1,1
%A A306448 _Jianing Song_, Feb 16 2019