A template for LaTeX cheat sheets

sheet.tex

% \documentclass[9pt,a4paper,twocolumn,landscape,oneside]{amsart}
\documentclass[9pt,a4paper,landscape,oneside]{amsart}
\usepackage{amsmath, amsthm, amssymb, amsfonts}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{booktabs}
\usepackage{fancyhdr}
\usepackage{float}
\usepackage{fullpage}
%\usepackage{geometry}
\usepackage[landscape]{geometry}
% \usepackage{listings}
\usepackage{caption, subcaption}
\usepackage[scaled]{beramono}
\usepackage{color,graphicx,overpic}
\usepackage{titling}
\usepackage{datetime}
\usepackage{multicol}
\usepackage{calc}
\usepackage{ifthen}
\usepackage{hyperref}
\usepackage{environ}


% Minted (For code stuff)
% \usepackage{minted}
% \newcommand{\code}[1]{\inputminted[fontsize=\footnotesize]{c}{#1}}


% This sets the margins to .5cm
\geometry{top=0pt,left=.3cm,right=.3cm,bottom=1cm}

\setlength{\headheight}{15.2pt}
\renewcommand{\headrulewidth}{0.4pt}
\renewcommand{\footrulewidth}{0.4pt}

% Turn off header and footer
% \pagestyle{empty}

% Redefine section commands to use less space
\makeatletter
\renewcommand{\section}{\@startsection{section}{1}{0mm}%
                                {-1ex plus -.5ex minus -.2ex}%
                                {0.5ex plus .2ex}%x
                                {\normalfont\large\bfseries}}
\renewcommand{\subsection}{\@startsection{subsection}{2}{0mm}%
                                {-1explus -.5ex minus -.2ex}%
                                {0.5ex plus .2ex}%
                                {\normalfont\normalsize\bfseries}}
\renewcommand{\subsubsection}{\@startsection{subsubsection}{3}{0mm}%
                                {-1ex plus -.5ex minus -.2ex}%
                                {1ex plus .2ex}%
                                {\normalfont\small\bfseries}}
\makeatother


% Don't print section numbers
\setcounter{secnumdepth}{0}


\setlength{\parindent}{0pt}
\setlength{\parskip}{0pt plus 0.5ex}

\newcommand{\subtitle}[1]{%
  \posttitle{%
    \par\end{center}
    \begin{center}\large#1\end{center}
    \vskip0.5em}%
}

% Header/Footer
% \geometry{includeheadfoot}
% \fancyhf{}
\pagestyle{fancy}
\lhead{Reykjavík University -  - Formúlublað}
\rhead{James Elías Sigurðarson\quad\thepage}
\cfoot{}

% Title/Author
\title{Formúlublað}
\subtitle{}
\date{\ddmmyyyydate{\today{}}}

% Output Verbosity
\newif\ifverbose
\verbosetrue
% \verbosefalse

% Some list helpers from graph theory
\newcounter{temp}
\newcounter{ilist_counter}
\newcounter{iilist_counter}

\newenvironment{ilist}{
  \begin{list}{{\bf \arabic{ilist_counter}}}{
      \usecounter{ilist_counter}
      \addtolength{\labelsep}{.6ex}
      \addtolength{\itemsep}{1ex}
      \setlength{\leftmargin}{1.4em}}
    %   \setcounter{ilist_counter}{\value{temp}}
}{
%   \setcounter{temp}{\value{ilist_counter}}  
  \end{list}
}

\newenvironment{iilist}{
  \begin{list}{{\bf \alph{iilist_counter}}}{
      \usecounter{iilist_counter}
      \addtolength{\labelsep}{.6ex}
      \addtolength{\itemsep}{.5ex}
      \setlength{\leftmargin}{1.7em}}
}{
  \end{list}
}

\newenvironment{iblist}{
  \begin{list}{{\bf $\bullet$}}{
      \addtolength{\labelsep}{.6ex}
      \addtolength{\itemsep}{.5ex}
      \setlength{\leftmargin}{1.7em}}
}{
  \end{list}
}

% Theorems and solutions
\theoremstyle{plain}
\newtheorem{theorem}{Theorem}
\newtheorem*{theorem*}{Theorem}
\newtheorem{corollary}[theorem]{Corollary}
\newtheorem*{corollary*}{Corollary}
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem*{lemma*}{Lemma}
\newtheorem{proposition}[theorem]{Proposition}
\newtheorem*{proposition*}{Proposition}
\newtheorem{conjecture}[theorem]{Conjecture}
\newtheorem*{conjecture*}{Conjecture}
\newtheorem*{solution}{Solution}

\theoremstyle{definition}
\newtheorem{definition}[theorem]{Definition}
\newtheorem*{definition*}{Definition}
\newtheorem{example}[theorem]{Example}
\newtheorem*{example*}{Example}
\newtheorem{problem}[theorem]{Problem}
\newtheorem*{problem*}{Problem}

\theoremstyle{remark}
\newtheorem{remark}{Remark}
\newtheorem*{remark*}{Remark}

% For writing vectors
\let\oldhat\hat
\let\oldvec\vec
\renewcommand{\vec}[1]{\oldvec{\mathbf{#1}}}
\newcommand{\vecb}[1]{\mathbf{#1}}
\renewcommand{\hat}[1]{\oldhat{\mathbf{#1}}}

\newcommand{\cvect}[2]{ \begin{pmatrix} #1 \\ #2 \end{pmatrix} }
\newcommand{\ctvect}[3]{ \begin{pmatrix} #1 \\ #2 \\ #3 \end{pmatrix} }
\newcommand{\vect}[2]{ \langle #1 , #2 \rangle }
\newcommand{\tvect}[3]{ \langle #1 , #2 , #3 \rangle }
\newcommand{\qvect}[4]{ \langle #1 , #2 , #3 \rangle }

% For equations
\NewEnviron{formula}{
    \abovedisplayshortskip=0pt
    \belowdisplayshortskip=0pt
    \abovedisplayskip=0pt
    \belowdisplayskip=0pt
    \begin{align*}
        \BODY
    \end{align*}
}
\newcommand{\eqn}[1]{\begin{formula} #1 \end{formula}}
% The actual document

\begin{document}
%\maketitle
\thispagestyle{fancy}
\raggedright
\footnotesize
\raggedcolumns
\begin{multicols*}{3}
\setlength{\premulticols}{1pt}
\setlength{\postmulticols}{1pt}
\setlength{\multicolsep}{1pt}
\setlength{\columnsep}{2pt}

\end{multicols*}
\end{document}