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• CommentAuthorUnknown G.
• CommentTimeMar 21st 2010

I would like to ask a question that includes some diagrams. I have tried using \xymatrix{...}, but that did not work. Is there any way to use xymatrix in MathOverflow?
1.

Not yet. jsMath, the engine we use to render LaTeX, doesn't support any commutative diagrams package. We'll switch to another engine, MathJax, once it is released. MathJax doesn't currently have support for commutative diagrams either, but I know that the developers want to add support for diagrams eventually. I believe (some subset of) xypic is the most likely candidate.

For now, I just use text-art diagrams, like this:

     f'
X --> Y
g'|     |g
v  f  v
Z --> W


and

  x ---> x
/|     /|
x ---> x |
| v    | v
| x ---|>x
v/     v/
x ---> x

2.
Alternatively, just make an image from the diagrams and paste it into the question using a file hosting service. I've used this for drawing subgraphs and Markov partitions in questions here, it works fine.
• CommentAuthorUnknown G.
• CommentTimeMar 21st 2010

Thank you very much, Anton and Steve!
3.

Might I suggest making the diagram using LaTeX matrices? Thse are a bit easier to read and give you some more flexibility, especiallly with labeling things.

• CommentAuthorUnknown G.
• CommentTimeMar 22nd 2010

4.
Note: You must surround your latex with backticks "" or else markdown will eat up all of your backslashes.
5.

Markdown will eat some backslashes, not all. That can actually be more confusing than if it ate them all, since it can be difficult to discern the pattern. So it is a good idea to use backticks around all mathematics.

• CommentAuthorHarry Gindi
• CommentTimeMar 22nd 2010 edited

It eats all backslashes before symbols, so \{ and \} will not appear at all, and if you only use one of them, jsMath will report an error that a close or open brace is missing.

Interestingly, since comments do not have markdown support, they render fine.
6.

I think Harald's point was that it only eats the first backslash when you type "\\", so you could not use backticks around a matrix, but use "\\\\" to start a new row. I prefer to use backticks any time I'd have to write improper LaTeX to get the right result otherwise.

7.
@Anton: Didn't you say at one point that there is a way for you to have it automatically put backticks around dollar signs?
8.

@fpqc: yes, in the sense that I've already overwritten the javascript for the editor. One day I might try to modify it so that it looks for math that might be messed up and put backticks around it. I'm not very excited about maintaining any code difference against the SE team, but it's probably possible to do it in a fairly clean way that won't be too hard to maintain.

9.

Slightly off topic, but I'd like to mention the fact that I never heard the word "backtick" before I came to this site. I guess I'd call that character an open quote mark or something, though frankly I've probably never called it anything. So it took me a while to figure out what people were going on about.

I think it would be helpful if wherever the word "backtick" appears in the help, there was a parenthetical comment such as "(the back-to-front apostrophe)" to explain what it means. (Or a link would do.)

10.

It can get a bit tricky to explain about the backtick (I think “backquote” is a more common name), since we want  but not ‘, both of which could fit the description.

11.
The symbol under the tilde.
• CommentAuthorMariano
• CommentTimeMar 22nd 2010

Tom, "backtick" is standard unix slang, although bash's manpage, for example, uses "backquote".

12.
• CommentAuthorHarry Gindi
• CommentTimeMar 22nd 2010 edited

http://en.wikipedia.org/wiki/Backtick

http://www.fileformat.info/info/unicode/char/0060/index.htm

Edit: Oops, meant to hit "edit post". Sorry for the double post!
13.

@fpqc: Too complicated. Anyhow, is there any keyboard that doesn't have a key with  on it? It's most likely a dead key on most European keyboards, so we might advice people with such keyboards to hit the space bar after it. Don't worry too much about the ‘ character – most people probably won't know how to type it anyhow.

• CommentAuthorHarry Gindi
• CommentTimeMar 22nd 2010 edited

Yes, but as Mariano pointed out, it's in a different spot on keyboards in different countries. In particular, on Mariano's keyboard (which I assume is the same as the other keyboard from Argentina that I found on google images), it is above the number "3".
14.

So we don't talk about its position on the keyboard. Just say what character we're talking about (), say it's somewhere on most (western) keyboards, and you may need to follow the key with a space to make it render properly. Leaver it at that. No need to tell them we're really talking about U+0060 GRAVE ACCENT. (Oh, but that is another name for it. The official name even.)