“…When ξ is a Markov diffusion, the above Zakai equation can also be obtained, under some additional hypotheses, when h is unbounded by means of the same arguments whenσ = 0, see (Baras, Blankenship, and Hopkins, 1983;Hopkins, 1982), and by different techniques whenσ does not depend on y and η is a onedimensional process, see (Florchinger, 1993). Furthermore, note that, under suitable hypotheses, a Zakai equation can be obtained when the state process ξ is a general Markov process (not necessarily given by (2)), and the observation process η is a diffusion process given by (3), with ω independent of ξ, see, e.g., (Bhatt, Kallianpur and Karandikar, 1995).…”