= Landau contour
{c}
{title2=$C_L$}
The velocity-integration contour implementing causal continuation of a kinetic response from the upper complex <phase velocity> half-plane. As a pole approaches the real axis, the contour passes below it. With denominator $v-u$, the real-axis limit is $\operatorname{PV}\int f_0'(v)/(v-u)\,dv+i\pi f_0'(u)$. Continuing the pole below the axis keeps this causal prescription rather than simply replacing the contour by an undeformed real-axis integral.
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