3. For which positive integers $n$ is $\phi(n)$
a) odd
b) divisible by 4
c) equal to $n / 2$ ?
3. a) 1,2 b) those integers $n$ such that $8|n ; 4| n$, and $n$ has at least one odd prime factor; n has at least two odd prime factors; or $n$ has a prime factor $\mathrm{p} \equiv 1(\bmod 4)$ c) $2^{k}, k=1,2, \ldots$