It has been conjectured that the
Fermat and Mersenne numbers are all square-free. In this note it is shown that if
some Fermat or Mersenne number fails to be square-free, then for any prime p whose
square divides the appropriate number, it must be that 2p−1≡ 1(modp2). At
present there are only two primes known which satisfy the above congruence. It is
shown that neither of these two primes is a factor of any Fermat or Mersenne
number.