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.

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

Offset: 2

Author

Felix Fröhlich, Mar 11 2015

Keywords

• nonn

Links

• Felix Fröhlich, Table of n, a(n) for n = 2..9999

Crossrefs

Cf. A242830.

Programs

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 *)

PARI

for(n=2, 120, i=0; forprime(p=1, n, if(Mod(n, p^2)^(p-1)==1, i++)); print1(i, ", "))