\begin
Discover why 18 million people worldwide trust Overleaf with their work.
\documentclass[a4paper,12pt]{report}
\usepackage[english]{babel}
\usepackage{subcaption}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{float}
\usepackage{lscape}
\usepackage{array}
\usepackage[matrix,arrow,curve]{xy}
\usepackage[T2A]{fontenc}
\usepackage[cp1251]{inputenc}
\usepackage{graphicx}
\usepackage{makecell}
\usepackage{titlesec}
\usepackage{titletoc}
\usepackage{blkarray}
\usepackage{rotating}
\usepackage{longtable}
\usepackage{geometry}
\usepackage{fancyhdr}
\usepackage{enumitem}
\newlist{steps}{enumerate}{1}
\setlist[steps, 1]{label = Step \arabic*:}
\usepackage{diagbox}
\usepackage{multirow}
\usepackage{url}
% \textwidth=175mm \textheight=257mm
% \voffset=-2.64cm \hoffset=-1.85cm
\geometry{tmargin=2cm,bmargin=2cm,lmargin=2.5cm,rmargin=1cm}
\fancyhf{}
\renewcommand{\headrulewidth}{0pt}
\fancyhead[C]{\thepage}
\pagestyle{fancy}
\fancypagestyle{plain}{%
\fancyhf{} % clear all header and footer fields
\renewcommand{\headrulewidth}{0pt}
\fancyhead[C]{\thepage} % except the center
}
\setlength{\headheight}{35pt}
\renewcommand{\thesection}{\arabic{chapter}.\arabic{section}}
\renewcommand{\thesubsection}{\arabic{chapter}.\arabic{section}.\arabic{subsection}}
\titleformat{\chapter}[display]{\bf\Huge\filcenter}{\chaptername\,\,\thechapter}{.5em}{}{}
\titlecontents{chapter}[0pt]{\filright}{\bf\chaptername\,\,\thecontentslabel \ }{\bf}{\dotfill\thecontentspage}
%\titleformat{\section}[block]{\bf\Large\filcenter}{\sectionname\,\,\thesection}{.5em}{}{}
\titlecontents{section}[1em]{\filright}{\thecontentslabel \ }{}{\dotfill\thecontentspage}
%\titleformat{\subsection}[block]{\bf\large}{\subsectionname\,\,\thesubsection}{.5em}{}{}
\titlecontents{subsection}[2em]{\filright}{\thecontentslabel \ }{}{\dotfill\thecontentspage}
\usepackage{xcolor,paracol}
\setlength{\parindent}{15pt}
\setcounter{page}{1}
\renewcommand{\baselinestretch}{1.2}
\newtheorem{acknowledgement}{Acknowledgement}
\newtheorem{definition}{Definition}[chapter]
\newtheorem{lemma}{Lemma}[chapter]
\newtheorem{theorem}{Theorem}[chapter]
\newtheorem{remark}{Remark}[chapter]
\newtheorem{proposition}{Proposition}[chapter]
\newtheorem{property}{chapter}
\newtheorem{corollary}{Corollary}[chapter]
\newtheorem{assumption}{Assumption}[chapter]
\newtheorem{example}{Example}[chapter]
\renewcommand{\thesection}{\arabic{chapter}.\arabic{section}}
\renewcommand{\theequation}{\arabic{chapter}.\arabic{equation}}
\newcommand{\secref}[1]{\S\ref{#1}}
\renewcommand*\footnoterule{}
\begin{document}
\input{Preface.tex}
\vfil\eject
\large \tableofcontents
\renewcommand{\partname}{Chapter}
\input{Introduction.tex}
\vfil\eject
\input{Chapter1.tex}
\vfil\eject
\input{Chapter2.tex}
\vfil\eject
\input{Chapter3.tex}
\vfil\eject
% \input{Chapter4.tex}
% \vfil\eject
\input{Conclusions.tex}
\vfil\eject
\input{Bibliography.tex}
\vfil\eject
\end{document}