When a Stochastic Exponential is a True Martingale. Extension of a Method of Bene^s

Abstract: Let $\mathfrak{z}$ be a stochastic exponential, i.e., $\mathfrak{z}t=1+\int_0^t\mathfrak{z}{s-}dM_s$, of a local martingale $M$ with jumps $\triangle M_t>-1$. Then $\mathfrak{z}$ is a nonnegative local martingale with $\E\mathfrak{z}t\le 1$. If $\E\mathfrak{z}_T= 1$, then $\mathfrak{z}$ is a martingale on the time interval $[0,T]$. Martingale property plays an important role in many applications. It is therefore of interest to give natural and easy verifiable conditions for the martingale property. In this paper, the property $\E\mathfrak{z}{_T}=1$ is verified with the so-called linear growth conditions involved in the definition of parameters of $M$, proposed by Girsanov \cite{Girs}. These conditions generalize the Bene^s idea, \cite{Benes}, and avoid the technology of piece-wise approximation. These conditions are applicable even if Novikov, \cite{Novikov}, and Kazamaki, \cite{Kaz}, conditions fail. They are effective for Markov processes that explode, Markov processes with jumps and also non Markov processes. Our approach is different to recently published papers \cite{CFY} and \cite{MiUr}.