Help drawing a figure with pgf

Here's an image of what I am trying to draw:

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Have you tried anything? If so, you should post a small document that shows your attempt. This shouldn't be to hard, and TikZ is well documented. –  Alan Munn Jun 6 '11 at 4:39
The title seems to indicate you want help, but your post gives the impression you want someone to do it for you. Which one is it? –  Vivi Jun 6 '11 at 6:17

This could probably be done more elegantly.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning,decorations.pathreplacing,calc}
\begin{document}
\begin{tikzpicture}[%
box/.style={draw,rectangle},
decoration={brace,amplitude=4}]
\node (posint) [box] {positive integer};
\node (negint) [box,right=of posint] {negative integer};

\draw [decorate,thick]
($(negint.south east) + (0,-.3)$) -- node (mida) {} ($(posint.south west) + (0,-.3)$);

\node (int) [box,below=.4cm of mida] {integer};
\node (frac) [box,right=of int] {fractional number};

\draw [decorate,thick]
($(frac.south east) + (0,-.3)$) -- node (midb) {} ($(int.south west) + (0,-.3)$);

\node (rational) [box,below=.4cm of midb] {rational number};

\draw [->] (posint) -- (negint);
\draw [->] (int) -- (frac);
\end{tikzpicture}
\end{document}

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This doesn't use pgf, sorry.

\documentclass[a4paper]{article}
\usepackage{amsmath}
\newcommand{\myline}[2]{\underbrace{\,#1\,}_{#2}}
\newcommand{\dblbox}[2]{\fbox{#1}\makebox[3em]{\rightarrowfill}\fbox{#2}}
\newcommand{\mytopstrut}{\vbox to 3ex{}}
\newcommand{\mybotstrut}{\vtop to 2ex{}}
\begin{document}
$\myline{\dblbox{positive integer}{negative integer}\mybotstrut}{ \hspace{11.5em}\myline{\dblbox{integer}{fractional number}\mybotstrut\mytopstrut}{ \fbox{rational number}\mytopstrut } }$
\end{document}

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Much as it loathes me to say it, not everything has to be done using TikZ! Nice solution. –  Loop Space Jun 6 '11 at 18:55

Egreg did what I had in mind to do. I need a mobile TeX :-)
Oh well, this one's in eplain, so hopefully there is enough difference:

\input eplain
\def\mybox#1{\vcenter{\boxit{\hbox{#1}}}}
\def\To{\hbox to 3em{\rightarrowfill}}
$$\displaylines{ \underbrace{\mybox{positive integer}\To\mybox{negative integer}}\cr \hfil\underbrace{\mybox{integer}\To\mybox{fractional number}}\cr \hfil\mybox{rational number}}$$
\bye

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