# Simple but deadly

**Number Theory**Level 4

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.