\documentclass[14pt,reqno,oneside,intlimits]{extarticle}
\usepackage[cp1251]{inputenc}
\usepackage[T2A]{fontenc}
\usepackage[14pt]{extsizes}
\usepackage{amsthm}
\usepackage{amsmath}
\DeclareMathOperator{\rank}{rank}
\makeatletter
\newenvironment{sqcases}{%
\matrix@check\sqcases\env@sqcases
}{%
\endarray\right.%
}
\def\env@sqcases{%
  \let\@ifnextchar\new@ifnextchar
  \left\lbrack
  \def\arraystretch{1.2}%
  \array{@{}l@{\quad}l@{}}%
}
\makeatother
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{dsfont}
\usepackage{ccaption}
\usepackage{amsfonts}
\usepackage{mathrsfs}
\usepackage[english,russian]{babel}
\usepackage{graphicx}
\usepackage{srcltx}
\usepackage{float}
\usepackage{tikz}
\usepackage[sf,bf,compact,topmarks,calcwidth,pagestyles]{titlesec}
\usepackage[colorlinks,urlcolor=blue]{hyperref}
\usepackage{geometry}
\geometry{left=2.5cm}
\geometry{right=1.5cm}
\geometry{top=2cm}
\geometry{bottom=2cm}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\fancyfoot[R]{\thepage}
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0pt}
\setcounter{page}{1}
\numberwithin{equation}{section}
\RequirePackage{caption}
\DeclareCaptionLabelSeparator{dotnspace}{. }
\captionsetup{justification=centering,labelsep=dotnspace}

\baselineskip 1.5 true pc
\newenvironment{comment}{}{}
 \newtheorem{thrm}{Òåîðåìà}[section]
\newenvironment{Proof}
{\par\noindent{\bf Äîêàçàòåëüñòâî.}}
{\hfill$\scriptstyle\blacksquare$}
\newtheorem{utr}{Óòâåðæäåíèå}[section]
\newtheorem{propos}{Ïðåäëîæåíèå}[section]
\newtheorem{cor}{Ñëåäñòâèå}[section]
\newtheorem{remrk}{Çàìå÷àíèå}[section]
\newtheorem{Def}{Îïðåäåëåíèå}[section]
\newtheorem{Example}{Ïðèìåð}[section]
\newtheorem{lem}{Ëåììà}[section]
\makeatletter
\renewcommand{\@biblabel}[1]{#1}
\makeatother
\renewcommand{\thesection}{\arabic{section}}
\makeatletter
\renewcommand{\section}{\@startsection{section}{1}%
{0.7cm}{3.5ex \@plus 1ex \@minus .2ex}%
{2.3ex \@plus .2ex}{\raggedright\normalfont\large\textbf}}
\makeatother
\makeatletter
\renewcommand{\subsection}{\@startsection{subsection}{2}%
{0.7cm}{3.5ex \@plus 1ex \@minus .2ex}%
{2.3ex \@plus .2ex}{\raggedright\normalfont\large\textbf}}
\makeatother