deletions | additions       diff --git a/introduction.tex b/introduction.tex  index 92f13cb..480c436 100644  --- a/introduction.tex  +++ b/introduction.tex 
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\begin{equation}  \sigma = \sigma_{dc}(V_{dc},V_{ac},\omega) + \sigma_{ac}(V_{dc},V_{ac},\omega)   \end{equation}  what is the expression of the  \begin{enumerate}  \item DC conductivity in the absence of AC voltage, i.e. $\sigma_{dc}(V_{dc},V_{ac}=0)$ voltage ($\sigma_{dc}(V_{dc},V_{ac}=0)$)  ? \item AC conductivity in the absence of AC voltage, i.e. $\sigma_{ac}(V_{dc}=0,V_{ac},\omega)$ voltage ($\sigma_{ac}(V_{dc}=0,V_{ac},\omega)$)  ? \itemThe  conductivity of the combined AC and DC voltage? \end{enumerate}