The Riemann zeta function ζ(s) = ∑ 1/n^s has an Euler product:
ζ(s) = ∏_p 1/(1+p^{-s})
ζ(s) = ∏_p 1/(1-p^{-s})
ζ(s) = ∑_p p^{-s}
ζ(s) = ∏_p (1-p^{-s})