A. Schlafly et S. Wagon, CARMICHAEL CONJECTURE ON THE EULER FUNCTION IS VALID BELOW 10(10,000,000), Mathematics of computation, 63(207), 1994, pp. 415-419
Carmichael's conjecture states that if phi(x) = n, then phi(y) = n for
some y not-equal x (phi is Euler's totient function). We show that th
e conjecture is valid for all x under 10(10,900,000). The main new ide
a is the application of a prime-certification technique that allows us
to very quickly certify the primality of the thousands of large numbe
rs that must divide a counterexample.