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How come when I use this code with beamer

 $$ \cal{M} = \frac{1-\lambda}{2}\sigma_v^2 M + \frac{1}{2(1-\lambda)}tr[\mathbf{RQ}] $$

results in this:

enter image description here

But when I remove the \cal, the equation is displayed correctly.

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\cal is a font, so you can also write your equation with $${\cal M}=\frac{1-\lambda}{2} ...$$ – lvaneesbeeck Dec 26 '13 at 0:32
up vote 8 down vote accepted

You need to use \mathcal, which takes an argument, not \cal:

enter image description here

\documentclass{beamer}% http://ctan.org/pkg/beamer
  \mathcal{M} = \frac{1-\lambda}{2}\sigma_v^2 M + \frac{1}{2(1-\lambda)}tr[\mathbf{RQ}]

The use is similar to your font switching macro \mathbf. Also note the use of \[...\] to denote a display math equation rather than the TeX $$...$$ notation. For more on this, see Why is \[ … \] preferable to $$?.

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