Function returns the rTPVar, defined in Andersen et al. (2012).
Assume there is \(N\) equispaced returns in period \(t\). Let \(r_{t,i}\) be a return (with \(i=1, \ldots,N\)) in period \(t\).
Then, the rTPVar is given by
$$
\mbox{rTPVar}_{t}=\frac{N}{N-2} \left( 2^{1/3} \frac{\Gamma \left(5/6\right)}{ \Gamma \left(1/2\right)} \right)^{-3} \sum_{i=3}^{N} \mbox({|r_{t,i}|}^{2/3} {|r_{t,i-1}|}^{2/3} {|r_{t,i-2}|}^{2/3})
$$