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\sect{Nyquist theory for a ring RL-circuit}
Derive the Nyquist expression for the current-current correlation
function in a closed ring, taking into account its inductance. Use
the following procedure:
\begin {itemize}
\item[(a)]
Write the R-L circuit equation for the current I, where the flux
$\Phi(t)$ through the ring is the driving parameter.
\item[(b)]
Identify the generalized susceptibility, and observe that it is
formally the same expression as in the problem of Brownian motion.
\item[(c)]
Calculate the current-current correlation function taking the
classical / high temperature limit.
\end {itemize}