User blog comment:Emlightened/Competition - Nov 2018/@comment-27513631-20181107231112/@comment-35470197-20181108232144

I see. So the variable \(n\) in \(f_{\infty}(n)\) varies only in \(\mathbb{N}\) while the variable \(n\) in \(f_n(k)\) varies in \(\mathbb{N} \cup \infty\). It is better to distinguish the symbols for variables which vary in distinct ranges next time, or clarify the ranges.