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

Author

Felix Fröhlich, Mar 23 2015

Keywords

• nonn

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

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).

Programs

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