# Commutative Diagram with Elements

I want to make a diagram that looks like the following:

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I suppose the things in the middle are turned ∈s, and not downward arrows? – Caramdir Apr 7 '12 at 19:34
Can you please search for Commutative Diagram on this site and tell us where exactly you are stuck with those answers? – percusse Apr 7 '12 at 19:40
Here is a good example Commutative Diagrams and TikZ that should get you started. – Peter Grill Apr 7 '12 at 20:10

There is a not so complicated answer using the packages graphicx and xypic. Before explaining I will straight away give my solution. It is the following:

\usepackage{graphicx}
\usepackage{xypic}
\usepackage{amsmath}

\begin{align*}
\xymatrix@R=10pt{
cRing \ar[r] & Sch \\
A \ar@{}[u]|{\rotatebox{90}{$\in$}} \ar@{|->}[r]
& Spec(A) \ar@{}[u]|{\rotatebox{90}{$\in$}}
}
\end{align*}


NOTE that diagram should of course be enclosed in \begin{document} and \end{document}. Of course it is not important that align* is used, any display-style math envisonment will do. The package amsmath is also just included to make align* available, since it is my favorite.

Now for the explanation. I will assume that you already know a little about the xypic. This package is really easy to use on a basic level. Here it has been used in a little more advanced way. The option @R=10pt between \xymatrix and the following { is an option the specifies the 'row spacing', and in this case we want it to be small so that the picture does not look silly. The top arrow is a standart arrow in xypic drawn with \ar[r] but the lower arrow has the extra symbols @{|->} which just specifies that we want this arrow to have that special look. In genneral it you want to rotate a symbol in LaTeX the graphicx package includes the command \rotatebox which allows for rotating element, e.g. the symbol \in. These symbols are placed like arrows with the command

\ar@{}[u]|{\rotatebox{90}{$\in$}}


Now \rotatebox{90}{$\in$} is jost a 90 degrees rotation of the symbol \in. The symbols @{} specifies that we want an 'empty' arrow, i.e. an arrow with no lines, and the symbols |{\rotatebox{90}{$\in$}} specify that we want \rotatebox{90}{$\in$} places as a lable on top of the given arrow.

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instead of align you should be able to just use $...$. – barbara beeton Apr 7 '12 at 21:51
Don't use align for a single equation. – egreg Apr 7 '12 at 23:35
I'm curious, why? – Kristian Apr 8 '12 at 0:34
\documentclass{article} \usepackage{tikz} \usetikzlibrary{matrix,arrows} \newcommand{\mapdef}[4]{ \begin{tikzpicture} \node (A) at (0,0) {#1}; \node (B) at (2,0) {#2}; \node (C) at (0,-1) {#3}; \node (D) at (2,-1) {#4}; %\path[|->,font=\scriptsize] \draw[->] (A) edge node[above] {} (B); \draw[|->] (C) edge node[above] {} (D); \draw[transparent] (A) edge node[rotate=90,opacity=1] {$\in$} (C); \draw[transparent] (B) edge node[rotate=90,opacity=1] {$\in$} (D); \end{tikzpicture} } \begin{document} $$\mapdef{A}{B}{a}{b}$$ \end{document} – user13372 Apr 8 '12 at 0:58
@user13372: What does this comment represent? Your current solution? – Werner Apr 8 '12 at 14:48