For which positive integers n do there exist integers x and y such with (x, n) = 1, (y, n) = 1, such that ?
It is easy to see that n = 2 satifies the given congruence. If n is odd, every prime p dividing n should satisfy .
The answer given in the book is: where = 0 or 1, the primes p in the product are all , and .
How do they get the in the product?