Edit: The quoted question addresses only numbers of the form $p^a q^b$, I asked a general question for arbitrary $n$.
If $n$ is a prime or a product of 2 primes then knowing its totient $\varphi(n)$ allows us immediately to find the prime factorization of $n$.
How about a general case? Does knowing $\varphi(n)$
- give us a way how to find the prime factorization of $n$,
- help as find a prime factor of $n$, or at least
- help at in finding any factor of $n$? (This turns out to be obvious.)
3 Answers
$\begingroup$In fact, it is a well-known observation in cryptography that just knowing a multiple of $\phi(n)$ helps greatly in factoring $n$, regardless of the number of prime factors (if there's only one or two primes dividing $n$ then this has already been covered).
Suppose $m$ is a multiple of $\phi(n)$. Then if you factor out enough powers of $2$ from $m$, there must exist a divisor $t = m/(2^r)$ such that $\lambda(n) \mid 2t$ but $\lambda(n) \nmid t$. (Here, $\lambda(n)$ is the Carmichael function, which will necessarily be even, unless $n$ is trivially small.)
It will then be the case that for some bases $b$, $b$ will be a quadratic residue for some prime $p \mid n$ but not for a different $q \mid n$. In this case, taking the GCD $(b^t-1,n)$ will produce a non-trivial factor of $n$.
One simply has to randomly try different values of $b$ (the expected number of tries is finite), as well as different choices of $t$ (there are at most $\log(n)$ possibilities, and one can use successive squaring to efficiently cover them).
To get all the prime factors of $n$, just keep repeating the process (we still have a multiple of $\phi$ for each of factors found above).
$\endgroup$ 7 $\begingroup$In the case of a prime, you can just observe that $\phi(n)=n-1$ to know it is prime. If $n=pq$ is a product to two primes, $\phi(n)=(p-1)(q-1)$, which you still need to factor. If you already know it is the product of two primes, you can use $\phi(n)=n-p-q+1$ to get $p+q$ as a second equation. As $\phi(n)$ has at least a couple factors of $2$ and may have other small factors, it will be somewhat smaller and can be easy to factor. Hagen von Eitzen makes a good point in the comment. If $n=pqr$, all prime, $\phi(n)=(p-1)(q-1)(r-1)$ and I don't see a good way to make headway except looking for small factors.
$\endgroup$ 4 $\begingroup$If $p^e$ divides $n$ then $p^{e-1}$ divides $\phi(n)$.
$\endgroup$
