\sect{Baruch's  D28.}

\begin{itemize}
\item[(a)]Write the Diffusion constant ${D}$ in terms of the
velocity-velocity correlation function. [Assume that this
correlation has a finite range in time].
\item [(b)]
Use Kubo's formula, assuming uncorrelated particles, to
derive the Einstein-Nernst formula for the mobility ${\mu =
eD/k_{B}T}$. ${[\mu  =\sigma \left(\omega =0\right)/ne }$ and
${n}$ is the particle density].\\
\end{itemize}

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