MathDB
Miklós Schweitzer 1959- Problem 5

Source:

October 30, 2015
college contests

Problem Statement

5. Denote by cnc_n the nnth positive integer which can be represented in the form cn=kl(k,l=2,3,)c_n = k^{l} (k,l = 2,3, \dots ). Prove that
n=11cn1=1\sum_{n=1}^{\infty}\frac{1}{c_n-1}=1
(N. 18)