[Tex/LaTex] How to get two mathrings centered over a variable

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I am trying to represent a second order, dimensionless derivative by two open circles overtop a variable. I understand how to do one (\mathring v), and I understand how to do multiple solid dots (\ddot v), but I can't figure out how to get two open circles over my variable. Any help would be greatly appreciated.

Best Answer

A modification of the \dddot macro of amsmath:

\documentclass{article}

\newcommand\ringring[1]{%
  {% make an Ord atom
   \mathop{\kern0pt #1}\limits^{% set a box over the variable
     \vbox to-1.85ex{
       \kern-2ex % lower the ring accents
       \hbox to 0pt{\hss\normalfont\kern.1em \r{}\kern-.45em \r{}\hss}%
       \vss % fill
     }% end of \vbox
   }% end of the superscript
  }% end of \mathop
}

\begin{document}
$\mathring{x}\ringring{x}\ddot{x}$
\end{document}

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