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 A255923 Bases b for which exactly two Wieferich primes p with p < b exist such that p is a base-b Wieferich prime. 11
 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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified June 16 15:06 EDT 2021. Contains 345057 sequences. (Running on oeis4.)