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URL: https://oeis.org/A306449

⇱ A306449 - OEIS

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A306449
Pseudoprimes to base 10 that are not squarefree.
2
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
1,1
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
PROG
(PARI) for(n=1, 10^6, if(Mod(10, n)^(n-1)==1 && !issquarefree(n), print1(n, ", ")))
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.
Sequence in context: A147829 A116274 A116286 * A160946 A359078 A100681
KEYWORD
nonn
AUTHOR
Jianing Song, Feb 16 2019
STATUS
approved