Let $$u=2^{2015}-1$$. Evaluate the congruence

$\varphi\left(u\right) \pmod{2015}$

where $$\varphi(u)$$ is the Euler totient function.

Tip: You probably don't want to compute $$\varphi(u)$$ because that would be, well, rather large -- there is a nice way around it.

×