A306449 Pseudoprimes to base 10 that are not squarefree.

9, 99, 657, 909, 1233, 11169, 13833, 19503, 20961, 23661, 51291, 69921, 90009, 99297, 109737, 139329, 203841, 237169, 256059, 321201, 339021, 346473, 460251, 475641, 686169, 760761, 927081, 1080801, 1621089, 1679931, 3100833, 3316941, 3845601, 3846051, 3942657, 4095081, 4281057

Offset: 1

Author

Jianing Song, Feb 16 2019

Keywords

• nonn

Comments

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

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

Intersection of A005939 and A013929.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..1000

Crossrefs

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).

Cf. also A045616, A005939, A013929.

Programs

PARI

for(n=1, 10^6, if(Mod(10, n)^(n-1)==1 && !issquarefree(n), print1(n, ", ")))