\documentclass[12pt]{article}
\usepackage{amsmath,amssymb,amsfonts}
\begin{document}
A graph $G$ with no isolated vertex is total domination bicritical if the removal of any pair of vertices, whose removal does not produce an isoï¿½lated vertex, decreases the total domination number. In this paper we study properties of total domination bicritical graphs, and give several characterizations.
\end{document}