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I would like to typeenter image description here

how do I do it?

Thanks!

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marked as duplicate by Gonzalo Medina, dustin, Werner, Sean Allred, cmhughes Sep 2 '13 at 4:07

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1 Answer

up vote 1 down vote accepted

Provided you've loaded the amsmath and amsfonts packages, you can write something like

\[
  \left. 
    \frac{\mathrm{d}\mathbb{P}^{\beta}}
         {\mathrm{d}\mathbb{P}} 
  \right|_{\mathcal{F}_t}
\]

enter image description here

\left and \right allows you to using grouping-like symbols and resize them to match the height of the material bracketed. In this case, you only have | which is not paired with anything else. So we use \left. to fill in for the missing grouping symbol.

Regarding \[...\], this is for displayed math expressions. It is currently preferred over $$...$$ as it handles various spacing issues better, among other things.

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I trust you that this works and i will use it but can i ask how exactly does this make the | long? –  Lost1 Sep 2 '13 at 0:42
    
you want the t outside the mathcal –  Lost1 Sep 2 '13 at 0:44
    
also what is [ ]? never used this before –  Lost1 Sep 2 '13 at 0:44
    
thanks a lot, really appreciate it. –  Lost1 Sep 2 '13 at 0:49
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