Continuity properties and the support of killed exponential functionals

Abstract

For two independent L\'evy processes $\xi$ and $\eta$ and an exponentially distributed random variable $\tau$ with parameter $q>0$, independent of $\xi$ and $\eta$, the killed exponential functional is given by $V_{q,\xi,\eta} := \int_0^\tau \mathrm{e}^{-\xi_{s-}} \, \mathrm{d} \eta_s$. Interpreting the case $q=0$ as $\tau=\infty$, the random variable $V_{q,\xi,\eta}$ is a natural generalization of the exponential functional $\int_0^\infty \mathrm{e}^{-\xi_{s-}} \, \mathrm{d} \eta_s$, the law of which is well-studied in the literature as it is the stationary distribution of a generalised Ornstein-Uhlenbeck process. In this paper, the support and continuity of the law of killed exponential functionals is characterized, and many sufficient conditions for absolute continuity are derived. We also obtain various new sufficient conditions for absolute continuity of $\smash{\int_0^te^{-\xi_{s-}}\mathrm{d}\eta_s}$ for fixed $t\geq0$, as well as for integrals of the form $\smash{\int_0^\infty f(s) \, \mathrm{d}\eta_s}$ for deterministic functions $f$. Furthermore, applying the same techniques to the case $q=0$, new results on the absolute continuity of the improper integral $\int_0^\infty \mathrm{e}^{-\xi_{s-}} \, \mathrm{d} \eta_s$ are derived. We also show that the law of the killed exponential functional $V_{q,\xi,\eta}$ arises as a stationary distribution of a solution to a certain stochastic differential equation, thus establishing a close connection to generalised Ornstein-Uhlenbeck processes