I don't know where to start.

$\displaystyle \sigma(n)$ is the sum of the divisors of n, including n itself.

Prove that there are no solutions to $\displaystyle \sigma(n)=17$.

I was thinking of using the formula $\displaystyle \sigma(p^a)=\frac {p^{a+1}-1}{p-1}$ where p is prime and $\displaystyle a\ge1$ to show that it's impossible to get 17, but I'm not sure if this is the right track.