A306449 - OEIS

%I #15 Mar 06 2021 06:00:19

%S 9,99,657,909,1233,11169,13833,19503,20961,23661,51291,69921,90009,

%T 99297,109737,139329,203841,237169,256059,321201,339021,346473,460251,

%U 475641,686169,760761,927081,1080801,1621089,1679931,3100833,3316941,3845601,3846051,3942657,4095081,4281057

%N Pseudoprimes to base 10 that are not squarefree.

%C Numbers k that are not squarefree and satisfy 10^(k-1) == 1 (mod k).

%C Any term is divisible by the square of a base-10 Wieferich prime (A045616 = {3, 487, 56598313, ...}).

%C Intersection of A005939 and A013929.

%H Amiram Eldar, <a href="/A306449/b306449.txt">Table of n, a(n) for n = 1..1000</a>

%o (PARI) for(n=1, 10^6, if(Mod(10, 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), A306448 (b=9), this sequence (b=10).

%Y Cf. also A045616, A005939, A013929.

%K nonn

%O 1,1

%A _Jianing Song_, Feb 16 2019