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\documentclass{math215}

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% following to use Times as the default font insteand of
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\usepackage{times,txfonts}

% The following commands set up the material that appears
% in the header.
\doclabel{Math 215: Homework 2}
\docauthor{Your name here.}
\docdate{February 4, 2013}

% I've provided a file (math215extras.tex) with some commonly used extra 
% commands. If you've downloaded it, you can include it in your document
% by uncommenting the line below.  Feel free to make changes to that file.
\input{math215extras.tex}

%%%% Main document starts here.

\begin{document}

\begin{proposition}{1.16}  If $m$ and $n$ are even integers, then so
  is $m+n$.
\end{proposition}
\begin{pf}
Your proof goes here.  For this one, please try to use the technique 
that I'll introduce on Monday of reminding the reader of the definition
of divisibility early in the proof.  ``To show that $m+n$ is even we must show that $\ldots$''.
\end{pf}

\begin{proposition}{1.17(ii) (Our version)} If $m$ is an integer
  and $m$ is divisible by $0$, then $m=0$.
\end{proposition}
\begin{pf}
Your proof goes here.
\end{pf}

\begin{proposition}{1.18}
Let $x\in\Ints$.  If $x$ has the property that for all $m\in\Ints$,
$mx=m$, then $x=1$.
\end{proposition}
\begin{pf}
Your proof goes here.
\end{pf}

\begin{proposition}{1.19}
Let $x\in\Ints$.  If $x$ has the property that for some nonzero $m\in\Ints$,
$mx=m$, then $x=1$.
\end{proposition}
\begin{pf}
Your proof goes here.
\end{pf}

\begin{proposition}{1.24} Let $x\in\Ints$.  If $x\cdot x= x$ then $x=0$ or $x=1$.
\end{proposition}
\begin{pf}
Your proof goes here.
\end{pf}

\begin{proposition}{1.25(i)} For all $m,n\in\Ints$
\[
-(m+n) = (-m) + (-n).
\]
\end{proposition}
\begin{pf}
Your proof goes here.
\end{pf}

\begin{proposition}{1.25(ii)} For all $m\in\Ints$
\[
-m = (-1)\cdot m.
\]
\end{proposition}
\begin{pf}
Your proof goes here.
\end{pf}


\end{document}