In this paper, we extend the 3/2-model for VIX studied by Goard and Mazur
(2013) and introduce the generalized 3/2 and 1/2 classes of volatility
processes. Under these models, we study the pricing of European and American
VIX options and, for the latter, we obtain an early exercise premium
representation using a free-boundary approach and local time-space calculus.
The optimal exercise boundary for the volatility is obtained as the unique
solution to an integral equation of Volterra type.
We also consider a model mixing these two classes and formulate the
corresponding optimal stopping problem in terms of the observed factor process.
The price of an American VIX call is then represented by an early exercise
premium formula. We show the existence of a pair of optimal exercise boundaries
for the factor process and characterize them as the unique solution to a system
of integral equations