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 A255920 Number of primes p with p < n such that n^(p-1) == 1 (mod p^2) i.e., number of Wieferich primes to base n less than n. 14
 0, 0, 0, 1, 0, 1, 1, 1, 1, 0, 0, 1, 0, 0, 0, 2, 2, 3, 0, 1, 1, 1, 1, 1, 2, 1, 3, 1, 1, 1, 1, 1, 0, 1, 0, 2, 1, 0, 2, 2, 1, 1, 1, 1, 1, 0, 1, 2, 1, 2, 0, 3, 1, 1, 0, 2, 0, 0, 1, 1, 2, 2, 1, 2, 0, 2, 3, 2, 1, 2, 0, 2, 1, 2, 2, 1, 1, 1, 3, 2, 2, 0, 0, 1, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,16 LINKS Felix Fröhlich, Table of n, a(n) for n = 2..9999 MATHEMATICA f[n_] := Block[{p = Complement[Prime@ Range@ PrimePi@ n, First /@ FactorInteger@ n]}, Select[p, Divisible[n^(# - 1) - 1, #^2] &]]; Length /@ Table[f@ n, {n, 2, 120}] (* Michael De Vlieger, Sep 24 2015 *) PROG (PARI) for(n=2, 120, i=0; forprime(p=1, n, if(Mod(n, p^2)^(p-1)==1, i++)); print1(i, ", ")) CROSSREFS Cf. A242830. Sequence in context: A187096 A340146 A340143 * A160115 A139365 A071479 Adjacent sequences: A255917 A255918 A255919 * A255921 A255922 A255923 KEYWORD nonn AUTHOR Felix Fröhlich, Mar 11 2015 STATUS approved

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Last modified September 29 02:13 EDT 2023. Contains 365747 sequences. (Running on oeis4.)