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I am trying to draw an arrow going down similar to these we usually see in topolgy books. For example, when we study covering spaces we usually put the covering space and then an arrow and then the base space.

Any help is appreciated.

Here is a picture:

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A simple down arrow is just \downarrow? Without an image of what you are looking for it is difficult to help you further. For complicated diagrams you should look into tikz. –  Peter Grill Feb 7 '12 at 2:27
    
I added an image. –  yaa09d Feb 7 '12 at 2:30
    
Are you interested in a "symbol" that you could use in inline text? Or are you interested in using this as a graphic in an image? For example, consider the multitude of different notations/forms available from the Wikipedia entry on Covering Space. –  Werner Feb 7 '12 at 2:43
    
If one of the answers accomplishes what you want, consider accepting it. –  Psirus Feb 16 '12 at 16:16

3 Answers 3

up vote 4 down vote accepted

I support the use of TikZ for diagrams, but if you are looking for something slightly simpler, you could try the amscd package.

@>>> and @VVV make right and down arrows, respectively. You can also use @<<< and @AAA for left or up arrows. The arrow labels go between the first and second characters or between the second and third characters, depending on whether you want them above/left or below/right.

\documentclass{article}
\usepackage{amscd}
\begin{document}

\[
  \begin{CD}
    E @>\tilde{f}>> F\\
    @V\pi_{E}VV @VV\pi_{F}V\\
    M @>>f> N
  \end{CD}
\]

\end{document}

diagram

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This seems simpler. Thanks! –  yaa09d Feb 7 '12 at 21:05

Here's a pretty quick option using tikz

screenshot

Note in particular the use of the positioning library, which allows you to specify nodes in terms of relative positions to other nodes- see Section 5.2 of the tikz & pgf documentation for more details. I also set the arrows as stealth which is short for 'stealth fighter jets'.

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{positioning}

% set arrows as stealth fighter jets
\tikzset{>=stealth}

\begin{document}

\begin{center}
\begin{tikzpicture}
    \node (E) at (0,0) {$E$};
    \node[right=of E] (F) {$F$};
    \node[below=of F] (N) {$N$};
    \node[below=of E] (M) {$M$};
    \draw[->] (E)--(F) node [midway,above] {$\tilde{f}$};
    \draw[->] (F)--(N) node [midway,right] {$\pi_F$};
    \draw[->] (M)--(N) node [midway,below] {$f$};
    \draw[->] (E)--(M) node [midway,left] {$\pi_E$};
\end{tikzpicture}
\end{center}

\end{document}
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Thanks! Looks great. –  yaa09d Feb 7 '12 at 6:35

Old fashioned way with Xy-pic:

\documentclass{article}
\usepackage[all,cmtip]{xy}

\begin{document}
\xymatrix{
  E \ar[r]^{\tilde{f}} \ar[d]_{\pi_{E}} & F \ar[d]^{\pi_{F}} \\
  M \ar[r]_{f} & N
}
\end{document}

enter image description here

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2  
jmilne.org/not/CDGuide.html is what i refer to for this stuff; there are example outputs linked to the page –  wasteofspace Feb 7 '12 at 15:12

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