\documentclass[a4paper,11pt]{article}

\usepackage[T1,T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english,russian]{babel}

<ПАКЕТЫ>

\everymath{\displaystyle}

\input{./pre/_environments}
\input{./pre/_operators}

\begin{document}
    \include{./sections/titlepage}
    \newgeometry{margin=1.25in}
    \stepcounter{page}
    \tableofcontents
    \include{./sections/introduction}
    \include{./sections/chapter1}
    \include{./sections/chapter2}
    \include{./sections/chapter3}
    \include{./sections/conclusion}

    \cleardoublepage
    \phantomsection
    \bibliographystyle{alpha}
    \bibliography{references}
\end{document}