OFFSET
1,1
COMMENTS
In analogy to a Wieferich pair, a set of three primes p, q, r can be called a 'Wieferich triple' if its members satisfy either of the following two sets of congruences:
p^(q-1) == 1 (mod q^2), q^(r-1) == 1 (mod r^2), r^(p-1) == 1 (mod p^2)
p^(r-1) == 1 (mod r^2), r^(q-1) == 1 (mod q^2), q^(p-1) == 1 (mod p^2)
a(9) must have A253683(n) > 121637. - Felix Fröhlich, Jun 18 2016
a(12) must have A253683(n) > 5*10^6. - Giovanni Resta, Jun 20 2016
LINKS
Wikipedia, Wieferich pair
PROG
(PARI) forprime(p=1, , forprime(q=1, p, forprime(r=1, q, if((Mod(p, q^2)^(q-1)==1 && Mod(q, r^2)^(r-1)==1 && Mod(r, p^2)^(p-1)==1) || (Mod(p, r^2)^(r-1)==1 && Mod(r, q^2)^(q-1)==1 && Mod(q, p^2)^(p-1)==1), print1(q, ", ")))))
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Felix Fröhlich, Jan 09 2015
EXTENSIONS
a(8) from Felix Fröhlich, Jun 18 2016
Name edited by Felix Fröhlich, Jun 18 2016
a(9)-a(11) from Giovanni Resta, Jun 20 2016
STATUS
approved