For mathjax, I don't know. For LaTeX, you can load the accents package, and use \underaccent{tilde}{x}.
– BernardMar 23 '18 at 18:44
\usepackage{amsmath} and \( \underset{\sim}{x} \) would work. However, it would be overkill to usepackage amsmathjust for this effect.
– Andreas Storvik StraumanMar 23 '18 at 19:49
accentspackage, and use\underaccent{tilde}{x}. – Bernard Mar 23 '18 at 18:44\usepackage{amsmath}and\( \underset{\sim}{x} \)would work. However, it would be overkill to usepackageamsmathjust for this effect. – Andreas Storvik Strauman Mar 23 '18 at 19:49