# How to write equations in a post?

1. Jul 16, 2011

### jacdiam89

Hello! I'm new here, and i would like to konw how to write an equation in my post. For example, the Freidmann equation. Do i need a particular software or there is a function to do it while i'm writing the post in the forum? Wich are the steps? Thank you
Jacopo

(sorry for my italianenglish, i will improve writing here :) )

2. Jul 16, 2011

### Chronos

Last edited by a moderator: Apr 26, 2017
3. Jul 16, 2011

### jacdiam89

Ok thank you, now i will try to write the Friedmann equation...

4. Jul 16, 2011

### jacdiam89

Oh, good, i've found the $\Sigma$ button above the text box. When i press this button a box appears with all the symbols. Problem solved!

$(\frac{\dot{a}}{a})^{2}=(H_{0})^{2}[ \Omega_{0,r}(\frac{a}{a_{0}})^{-4}+\Omega_{0,m}(\frac{a}{a_{0}})^{-3}+(1-\Omega_{0})(\frac{a}{a_{0}})^{2}+\Omega_{\Lambda}]$

Et voilà, thank you again.

Jacopo

5. Jul 17, 2011

### Redbelly98

Staff Emeritus
Glad you found how to do it. FYI, we often point new members who ask this question to the following thread:

https://www.physicsforums.com/showthread.php?t=386951 [Broken]

Last edited by a moderator: May 5, 2017
6. Jul 17, 2011

### Fredrik

Staff Emeritus
That thread could use some cleanup. Borek's post about the old preview bug should be deleted. It's weird to explain multiple lines by linking to page 19 of 49 of an old locked thread filled with lots of irrelevant information and broken LaTex code. (I would just link to a web page that explains the align environment). Those are just examples. I'm reluctant to link to this thread myself because of all the problems with it.

The thread should at least explain the difference between itex and tex tags (you could e.g. use my example below), link to the comprehensive LaTeX symbol list, mention the 50 character bug, mention the quick edit refresh bug, and mention that the best way to learn is to click on the quote button every time you see a post that makes you think "I wonder how he did that". (OK, I see now that the "how to latex" document actually does that last thing).

Example of itex vs tex: The function $\sin:\mathbb R\rightarrow\mathbb R$ is defined by $$\sin x=\sum_{n=0}^\infty \frac{(-1)^n}{(2n+1)!}x^{2n+1},$$ for all $x\in\mathbb R$. Note that the comma must be placed inside the tex tags, and that you shouldn't type any line breaks.

The "how to latex" pdf should also be updated. It doesn't mention itex tags, fails to mention that \\ only works in specific environments (like align), falsely claims that tex tags are equivalent to "dollar sign, LaTeX code, dollar sign" in a LaTeX document. (This would give you a result equivalent to itex. You need two dollar signs before and after the code to get a result equivalent to tex). The pdf explains some things in awkward ways. For example matrices: $$\Lambda=\gamma\begin{pmatrix}1 & -v\\ -v & 1\end{pmatrix}$$ I also think people would prefer to use the cases environment instead of arrays: $$\chi_{\mathbb Q}(x)=\begin{cases}1 &\text{if }x\in\mathbb Q\\ 0 &\text{if }x\notin\mathbb Q.\end{cases}$$ The displaystyle command is explained badly. It's used to prevent LaTeX from formatting the math the way it normally would when the math image is to appear on a line that also includes text. Compare $\sum_{k=0}^n x_k$ to ${\displaystyle \sum_{k=0}^n x_k}$. The latter code includes a "displaystyle". Note the positions of the curly brackets. They don't go where you'd normally put them. (Compare it with the source code for this image: $\sqrt{x^2}$).

Last edited by a moderator: May 5, 2017
7. Jul 17, 2011

### Redbelly98

Staff Emeritus
I deleted Borek's post. The thread was originally meant to direct people to other areas of the forum that explained things in more detail. At some point I added a simple example to the post, so that people completely new to LaTeX would have a starting point.

I agree that a cleanup of the detailed how-to stuff would be a big help.

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