Nicer overline with index
% This fixes bad index positioning with overline
% https://tex.stackexchange.com/questions/346842/symbol-with-overline-and-subscript-leads-to-wrong-kerning
\newcommand\mean[2][]{%
\overline{#2}%
\ifx\relax#1\relax\else
\sbox0{$#2_{#1}$}%
\sbox2{$\overline{#2}_{#1}$}%
\kern\dimexpr\wd0-\wd2\relax_{#1}%
\fi%
}
% usage example
$\mean[0]{T}$