title: Reference an equation in a different file with Sphinx
body: <p>I am documenting my code using <a href="http://sphinx.pocoo.org/" rel="nofollow">Shinx</a>. In some files I have mathematical formulas and would like to include a reference to them. Being in the very same file this is not a problem. Using <code>:eq:'reference'</code>.</p>
  
  <p>However, how do you cross reference to a formula from a different file? In my case <a href="http://sphinx.pocoo.org/" rel="nofollow">Sphinx</a> is unable to find the reference.</p>