\documentclass[a4paper]{article}
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\newenvironment{proof}[1][Proof]{\noindent\textbf{#1.} }{\ \rule{0.5em}{0.5em}}
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\newtheorem{theorem}{\U{422}\U{435}\U{43e}\U{440}\U{435}\U{43c}\U{430}}% % Теорема
\newtheorem{lemma}{\U{41b}\U{435}\U{43c}\U{43c}\U{430}}% % Лемма
\newtheorem{corollary}{\U{421}\U{43b}\U{435}%
\U{434}\U{441}\U{442}\U{432}\U{438}\U{435}}% % Следствие
\newtheorem{definition}{\U{41e}\U{43f}\U{440}%
\U{435}\U{434}\U{435}\U{43b}\U{435}\U{43d}\U{438}\U{435}}% % Определение
\newtheorem{proposition}{\U{41f}\U{440}\U{435}%
\U{434}\U{43b}\U{43e}\U{436}\U{435}\U{43d}\U{438}\U{435}}% %Предложение
\newtheorem{remark}{\U{417}\U{430}\U{43c}%
\U{435}\U{447}\U{430}\U{43d}\U{438}\U{435}}% %Замечание
\begin{document}
\end{document}