OFFSET
1,1
COMMENTS
Any prime p divides Sum_{k=1..(p-1)/2} (k^(p-2))*(k^(p-1)-1). But a restricted list of primes p are such that p^2 divides Sum_{k=1..(p-1)/2}(k^(p-2))*(k^(p-1)-1).
Also primes p such that (2^(p-1)-1)/p == 0 (mod p) or 2*((p-1)!+1)/p +(2^(p-1)-1)/p == 0 (mod p), because it can be shown that Sum_{k=1..(p-1)/2} (k^(p-2))*(k^(p-1)-1) == p*((2^(p-1)-1)/p)*(2*((p-1)!+1)/p +(2^(p-1)-1)/p) (mod p^2).
The Wieferich primes (A001220) belong to the sequence.
LINKS
Amir Akbary and Sahar Siavashi, The Largest Known Wieferich Numbers, INTEGERS, 18(2018), A3. See Table 1 p. 5.
MATHEMATICA
p=3; While[p<20000, If[Mod[Sum[PowerMod[k, p-2, p^2]*(PowerMod[k, p-1, p^2]-1), {k, 1, (p-1)/2}], p^2] == 0, Print [p]]; p=NextPrime[p]]
PROG
(PARI) is(n)=if(!isprime(n), return(0)); my(m=n^2, e=n-2); sum(k=1, n\2, Mod(k, m)^e*(Mod(k, m)^(e+1)-1))==0 && n>2 \\ Charles R Greathouse IV, Nov 13 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
René Gy, Nov 11 2016
STATUS
approved