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A255925
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Bases b for which exactly four Wieferich primes p with p < b exist such that p is a base-b Wieferich prime.
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11
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116, 117, 118, 233, 245, 249, 251, 261, 269, 276, 298, 325, 369, 374, 401, 423, 460, 485, 487, 505, 526, 604, 618, 629, 653, 717, 721, 723, 737, 776, 793, 838, 851, 856, 857, 863, 867, 881, 893, 932, 962, 969, 978, 1025, 1037, 1045, 1057, 1059, 1079, 1106
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OFFSET
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1,1
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COMMENTS
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Numbers b such that A255920(b) = 4.
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LINKS
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MATHEMATICA
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wp[b_] := Count[Complement[Prime[Range[PrimePi[b]]], FactorInteger[b][[All, 1]] ], p_ /; Divisible[b^(p - 1) - 1, p^2]];
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PROG
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(PARI) is(n) = my(i=0); forprime(p=1, n-1, if(Mod(n, p^2)^(p-1)==1, i++); if(i > 4, return(0))); i==4
(Sage) [b for b in range(3, 1107) if len([p for p in range(2, b) if is_prime(p) and mod(b, p^2)^(p-1)==1])==4] # Danny Rorabaugh, Mar 31 2015
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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