Goldbach's original proof to Euler involved assigning a constant to the harmonic series: x = ∑ n = 1 ∞ 1 n {\displaystyle \textstyle x=\sum _{n=1}^{\infty }{\frac {1}{n}}} , which is divergent. Such a proof is not considered rigorous by modern standards. There is a strong resemblance between the method of sieving out powers employed in his proof and the method of factorization used to derive Euler's product formula for the Riemann zeta function.