%I #16 Mar 06 2021 06:00:30

%S 4,8,28,52,121,364,532,616,1036,1288,3052,3751,4376,4636,4961,5356,

%T 6364,7381,8744,11011,11476,12124,15964,19096,19684,21196,21736,24388,

%U 26596,29161,31876,32791,37576,40132,45676,47972,53092,61831,67276,72136,80476,80956,86296

%N Pseudoprimes to base 9 that are not squarefree.

%C Numbers k that are not squarefree and satisfy 9^(k-1) == 1 (mod k).

%C 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 Intersection of A020138 and A013929.

%H Amiram Eldar, <a href="/A306448/b306448.txt">Table of n, a(n) for n = 1..10000</a>

%o (PARI) for(n=1, 10^5, if(Mod(9, n)^(n-1)==1 && !issquarefree(n), print1(n, ", ")))

%Y 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 Cf. also A014127, A020138, A013929.

%K nonn

%O 1,1

%A _Jianing Song_, Feb 16 2019