A255923 Bases b for which exactly two Wieferich primes p with p < b exist such that p is a base-b Wieferich prime.

17, 18, 26, 37, 40, 41, 49, 51, 57, 62, 63, 65, 67, 69, 71, 73, 75, 76, 81, 82, 93, 97, 101, 109, 121, 124, 125, 127, 130, 134, 149, 157, 164, 165, 168, 172, 174, 179, 188, 190, 193, 195, 199, 201, 202, 206, 207, 214, 217, 218, 221, 229, 235, 236, 239, 241

Offset

1,1

Comments

Numbers b such that A255920(b) = 2.

Links

Felix Fröhlich, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Wieferich Prime

Prog

(PARI) is(n) = my(i=0); forprime(p=1, n-1, if(Mod(n, p^2)^(p-1)==1, i++); if(i > 2, return(0))); i==2
(Sage) [b for b in range(3, 242) if len([p for p in range(2, b) if is_prime(p) and mod(b, p^2)^(p-1)==1])==2] # Danny Rorabaugh, Mar 31 2015

Crossrefs

Cf. A255920.

Cf. bases b with exactly k base-b Wieferich primes less than b: A255921 (k=0), A255922 (k=1), A255924 (k=3), A255925 (k=4), A325881 (k=5), A325882 (k=6), A325883 (k=7), A325884 (k=8), A325885 (k=9), A325886 (k=10).

Sequence in context: A308888 A084379 A018821 * A226673 A151978 A111054

Adjacent sequences: A255920 A255921 A255922 * A255924 A255925 A255926

Keyword

nonn

Author

Felix Fröhlich, Mar 23 2015

Status

approved