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\begin{equation*}
\sum_{t} -\ln \left[\left<R(t)|\exp[-\tau\lambda\nabla^2]|R(t+\tau)\right>\right] = \text{const.} + \sum_{t,i} \frac{|r_{i,t}-r_{i,t+\tau}|^2}{4\lambda\tau} 
\end{equation*}
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