Euler's totient φ(n) counts how many positive integers below n are coprime to n.
You try
φ(12) = ? (how many of 1…11 share no factor with 12)
Try it
φ(10)
Coprime to 10 below it: 1, 3, 7, 9. So φ(10) = 4.
Used in RSA encryption — and famously, e^(iπ) + 1 = 0 lives in Euler's universe.