Since version 0.8 it is supported natively: You shouldn’t use any workaround anymore. The syntax is also very simple. It is the same as latex math, but without the enclosing $$
So you can simply write the following for a math block
.. math::
\frac{ \sum_{t=0}^{N}f(t,k) }{N}
Or if you want to write inline you can use this:
:math:`\frac{ \sum_{t=0}^{N}f(t,k) }{N}`
notice the delimiting backticks there.
UPDATE:
in newer versions it seems to be necessary to use a double-backslash for the math elements, so it’s \\frac and not \frac