What process is it when the polytropic index $n$ is equal to $- infty$?

begin{align} P v^{-infty} &= K frac{P}{v^infty} &= K frac{v^infty}{P} &= frac{1}{K} v^inftyfrac{1}{P} &= underbrace{frac{1}{K}}_{K'} end{align} Variations in $v$ get scaled by an exponent of $infty$ and are therefore infinitely more important in determining whether the result is $K'$ than variations in $1/P$ are. The process is still constant volume.