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Next: References Up: Kac's Moment Formula and Previous: Application to Markov Chains

Application to Local Times

  Suppose as in [8] (3.41) that X with state space E admits a jointly measurable local time process tex2html_wrap_inline3918 relative to a reference measure dx on E, so that for tex2html_wrap_inline3054

  equation2157

For example, X could be a Markov chain with countable state space, with tex2html_wrap_inline3784 , or a one-dimensional diffusion [56].

Fix a Markov killing time T for X. Then tex2html_wrap_inline3936 serves as a density for the Green's kernel of X killed at time T. From (22), for all tex2html_wrap_inline3942 there is the formula

  equation2172

So for each a such that tex2html_wrap_inline3950 the covariance

  equation2182

is a symmetric non-negative definite function of tex2html_wrap_inline3952 . Suppose now that a finite subset F of E is such that tex2html_wrap_inline3804 , for all tex2html_wrap_inline3960 . The results of Subsection 3.6 show that for any initial distribution tex2html_wrap_inline2820 on F the tex2html_wrap_inline2992 joint distribution of tex2html_wrap_inline3968 is determined by the values of the Green's function g(x,y) for tex2html_wrap_inline3972 . In particular, all product moments tex2html_wrap_inline3974 for non-negative integer n(y) have finite values which can be read from (27). And the tex2html_wrap_inline2992 joint moment-generating function of the tex2html_wrap_inline3968 converges in a neighborhood of the origin and is given there by the formula

  equation2197

where tex2html_wrap_inline3984 . Put another way, (57) states that for v in a neighborhood of tex2html_wrap_inline3988 , the function

displaymath3990

is the unique solution f of the system of equations

displaymath3994

For X a one-dimensional diffusion, Ray [55] (2.1) derived this system of equations for an h-process obtained conditioning on tex2html_wrap_inline4000 , and went on to show that these equations imply the Ray-Knight descriptions for the distribution of local times of one-dimensional diffusions stopped at a Markov killing time. See also Sheppard [61] who recovered most of Ray's results with the help of Dynkin's isomorphism theorem, and [53] for further discussion.

Note that the class of possible finite-dimensional distributions for tex2html_wrap_inline3968 as above is precisely the class of joint distributions of total occupation times of various states in a finite state Markov chain. This can be understood by considering the time-changed Markov chain tex2html_wrap_inline4004 where tex2html_wrap_inline4006 is the inverse of tex2html_wrap_inline4008 . Williams [65, 66] used a similar time change argument to derive variations of formula (55) for local time processes associated with both ordinary and fictitious states of a countable state Markov chain. See Theorem 6.1 of [66].

An altogether different application of the F-K formula to the local times of one-dimensional Lévy processes can be found in [6]. This work concerns the law of the Hilbert transform of tex2html_wrap_inline3794 with respect to x (for certain random T); it extends and simplifies [23], in which Kac's moment formula (4) is used. See also [31] regarding connections between the F-K formula and path decompositions for one-dimensional BM.

Acknowledgement. We are grateful to Jay Rosen for his illuminating comments on the manuscript.


next up previous
Next: References Up: Kac's Moment Formula and Previous: Application to Markov Chains

Patrick Fitzsimmons
Wed May 17 08:50:36 PDT 2000