Placing an identity directly beneath a symbol

Question

How do we we write in latex a phrase directly under another symbol. I want to get the r going to infinity directly beneath the Tilda as shown below . However am unable to do so

I tried using \sim_{\r \rightarrow \infty} but this does not work.

Answer 1

Just using \underset{below}{upper}...and I think that you can compile yourself the final code.

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\begin{document}
\[F_{ij}\underset{r\to\infty}{\sim}\frac{1}{\sqrt{k_j}}\]
\end{document}