
\documentclass{article}
% With T1 enabled, \"\i yields accented i, not bad sqrt.
% See https://tug.org/TUGboat/tb45-1/tb139beet.pdf
%%\usepackage[T1]{fontenc}
  \pagestyle{empty}
\begin{document}
\renewcommand{\i}{\ensuremath{\sqrt{-1}}}
Brezis, Ha\"{\i}m
\end{document}
