\documentclass[12pt]{article}

%New commands

\newcommand{\nat}{\mbox{${\mathbb N}$}}
\newcommand{\real}{\mbox{${\mathbb R}$}}
\newcommand{\integer}{\mbox{$\mathbb Z$}}
\newcommand{\rational}{\mbox{$\mathbb Q$}}

\newcommand{\power}[1]{{\cal P}(#1)}
\newcommand{\comp}[1]{#1^{\cal C}}

\newcommand{\forward}{\noindent $\Longrightarrow$}
\newcommand{\back}{\noindent $\Longleftarrow$}

\newenvironment{problem}[2]{\noindent {\bf Problem #1 in Chapter #2:}}
{\\
\medskip}

\newenvironment{theorem}[1]{\bigskip \noindent {\bf Theorem #1:}
\medskip}

\newenvironment{proof}{\noindent {\bf Proof:}}{\hfill
\rule{1mm}{3mm} \bigskip}

%Other important packages

\usepackage{graphicx}  %This one allows you to include graphics. Ask for details.
\usepackage{amssymb}   %This one includes some more symbols that you may want.

\begin{document}



\end{document}
