The Levy-Khintchine Formula

Because of a string of coincidences involving my choice of courses for Part III and various lecturers’ choices about course content, I didn’t learn what a Levy process until a few weeks’ ago. Trying to get my head around the Levy-Khintchine formula took a little while, so the following is what I would have liked to have been able to find back then.

A Levy process is an adapted stochastic process started from 0 at time zero, and with stationary, independent increments. This is reminiscent, indeed a generalisation, of the definition of Brownian motion. In that case, we were able to give a concrete description of the distribution of X_1. For a general Levy process, we have

X_1=X_{1/n}+(X_{2/n}-X_{1/n})+\ldots+(X_1-X_{1-1/n}).

So the distribution of X_1 is infinitely divisible, that is, can be expressed as the distribution of the sum n iid random variables for all n. Viewing this definition in terms of convolutions of distributions may be more helpful, especially as we will subsequently consider characteristic functions. If this is the first time you have seen this property, note that it is not a universal property. For example, it is not clear how to write a U[0,1] random variable as a convolution of two iid RVs. Note that exactly the same argument suffices to show that the distribution of X_t is infinitely divisible.

It will be most convenient to work with the characteristic functions

\mathbb{E}\exp(i\langle \lambda,X_t\rangle).

By stationarity of increments, we can show that this is equal to

\exp(-\Psi(\lambda)t)\quad\text{where}\quad \mathbb{E}\exp(i\langle \lambda,X_1\rangle)=:\exp(-\Psi(\lambda)).

This function \Psi(\lambda) is called the characteristic exponent. The argument resembles that used for Cauchy’s functional equations, by dealing first with the rationals using stationarity of increments, then lifting to the reals by the (right-)continuity of

t\mapsto \mathbb{E}\exp(i\langle \lambda,X_t\rangle).

As ever, \Psi(\lambda) uniquely determines the distribution of X_1, and so it also uniquely determines the distribution of Levy process. The only condition on \Psi is that it be the characteristic function of an infinitely divisible distribution. This condition is given explicitly by the Levy-Khintchine formula.

Levy-Khintchine

\Psi(\lambda) is the characteristic function of an infinitely divisible distribution iff

\Psi(\lambda)=i\langle a,\lambda\rangle +\frac12 Q(\lambda)+\int_{\mathbb{R}^d}(1-e^{i\langle \lambda,x\rangle}+i\langle \lambda,x\rangle 1_{|x|<1})\Pi(dx).

for a\in\mathbb{R}^d, Q a quadratic form on \mathbb{R}^d, and \Pi a so-called Levy measure satisfying \int (1\wedge |x|^2)\Pi(dx)<\infty.

This looks a bit arbitrary, so first let’s explain what each of these terms ‘means’.

  • i\langle a,\lambda\rangle comes from a drift of -a. Note that a deterministic linear function is a (not especially interesting) Levy process.
  • \frac12Q(\lambda) comes from a Brownian part \sqrt{Q}B_t.

The rest corresponds to the jump part of the process. Note that a Poisson process is an example of a Levy process, hence why we might consider thinking about jumps in the first place. The reason why there is an indicator function floating around is that we have to think about two regimes separately, namely large and small jumps. Jumps of size bounded below cannot happen too often as otherwise the process might explode off to infinity in finite time with positive probability. On the other hand, infinitesimally small jumps can happen very often (say on a dense set) so long as everything is controlled to prevent an explosion on the macroscopic scale.

There is no canonical choice for where the divide between these regimes happens, but conventionally this is taken to be at |x|=1. The restriction on the Levy measure near 0 ensures that the sum of the squares all jumps up some finite time converges absolutely.

  • \Pi\cdot 1_{|x|\geq 1} gives the intensity of a standard compound Poisson process. The jumps are well-spaced, and so it is a relatively simple calculation to see that the characteristic function is

\int_{\mathbb{R}^d}(1-e^{i\langle \lambda,x\rangle})1_{|x|\geq 1}\Pi(dx).

The intensity \Pi\cdot 1_{|x|<1} gives infinitely many hits in finite time, so if the expectation of this measure is not 0, we explode immediately. We compensate by drifting away from this at rate

\int_{\mathbb{R}^d}x1_{|x|<1}\Pi(dx).

To make this more rigorous, we should really consider 1_{\epsilon<|x|<1} then take a limit, but this at least explains where all the terms come from. Linearity allows us to interchange integrals and inner products, to get the term

\int_{\mathbb{R}^d}(1-e^{-i\langle \lambda,x\rangle}+i\langle\lambda,x\rangle 1_{|x|<1})\Pi(dx).

If the process has bounded variation, then we must have Q=0, and also

\int (1\wedge |x|)\Pi(dx)<\infty,

that is, not too many jumps on an |x| scale. In this case, then this drift component is well-defined and linear \lambda, so can be incorporated with the drift term at the beginning of the Levy-Khintchine expression. If not, then there are some \lambda for which it does not exist.

There are some other things to be said about Levy processes, including

  • Stable Levy processes, where \Psi(k\lambda)=k^\alpha \Psi(\lambda), which induces the rescaling-invariance property: k^{-1/\alpha}X_{kt}\stackrel{d}{=}X. The distribution of each X_t is then also a stable distribution.
  • Resolvents, where instead of working with the process itself, we work with the distribution of the process at a random exponential time.
Advertisements

Markovian Excursions

In the previous post, I talked about the excursions of a Brownian motion. Today I’m thinking about how to extend these ideas to more general Markov chains. First we want to rule out some situations. In particular, we aren’t hugely interested in discrete time Markov chains. The machinery is fairly well established for excursions, whether or not the chain is transient. Furthermore, if the state space is discrete, as for a Poisson process for example, the discussion is not hugely interesting either. Remember that the technical challenges in the constructions of local time arise because of the Blumenthal 0-1 law property that Brownian motion visits 0 infinitely often in the small window after the start time. We therefore want the process to be regular at the point of the state space under discussion. This is precisely the condition described above for BM about 0.

Why is it harder in general?

The informal notion of a local time should transfer to a more general Markov chain, but there are some problems. Firstly, to define something in terms of an integral is not general enough. The state space E needs some topological structure, but any meaningful definition must be in terms of functions from E into the reals. There were also all sorts of special properties of Brownian motion, including the canonical time-space rescaling that came in handy in that particular case. It turns out to be easiest to consider the excursion measure on a general Markov chain through its Laplace transform.

Definition and Probabilistic interpretations

The resolvent is the Laplace transform of the transition probability P_t(x,A), viewed as an operator on functions f:E\rightarrow \mathbb{R}.

R_\lambda f(x):=\mathbb{E}_x\left[\int_0^\infty e^{-\lambda t}f(X_t)dt\right]=\int_0^\infty e^{-\lambda t}P_tf(x)dt.

We can interpret this in terms of the original process in a couple of ways which may be useful later. The motivation is that we would like to specify a Poisson process of excursions, for which we need to know the rate. We hope that the rate will in fact be constant, so it will in fact to suffice to work out the properties of the expected number of excursions (or whatever) up to various random times, in particular those given by exponential RVs.

So, we take \zeta\sim\text{Exp}(\lambda) independent of everything else, and assume that we ‘kill the chain’ at time \zeta. Then, by shuffling expectations in and out of the integral and separating independent bits, we get:

R_\lambda f(x)=\mathbb{E}_x\int_0^\zeta f(X_s)ds = \frac{1}{\lambda}\mathbb{E}_xf(X_\zeta).

As in the Brownian local time description

R_\lambda 1_A(x)=\mathbb{E}(\text{time spent in }A\text{ before death at time }\zeta_\lambda).

Markovian property

We want to show that excursions are Markov, once we’ve sorted out what an ‘excursion’ actually means in this context. We do know how to deal with the Markovian property once we are already on an excursion. It is relatively straightforward to define an extension of the standard transition probability operator, to include a condition that the chain should not hit point a during the transition. That is

_aP_t(x,A):= \mathbb{P}_x(X_t\in A\cap H_a>t).

This will suffice to define the behaviour once an excursion has started. The more complicated bit will be the entrance law n_t(A), being the probability of arriving at A after time t of an excursion. To summarise, as with BM, all the technical difficulties with an excursion happen at the beginning, ie bouncing around the start point, but once it is ‘up-and-running’, its path is Markovian and controlled by _aP_t.

Marking

The link between the resolvent and the excursions, is provided as in the Brownian case, by supplying a PPP of marks at uniform rate \lambda to real time. This induces a mark process on excursions, weighted by an (exponential) function of excursion length. We make no distinction between an excursion including one mark or many marks. Then the measure on marked excursion is, in a mild abuse of notation:

n_\lambda=(1-e^{-\lambda\zeta(f)})\cdot n.

We compare with the Laplace transform n_\lambda(dx)=\int_0^\infty e^{-\lambda t}dtn_t(dx) using a probabilistic argument.

We can apply the measure to a function in the usual way: \lambda n_\lambda(1_A) is the measure of those excursions for which the first mark occurs in A. So by taking A=E, we get

\lambda n_\lambda(1)=\text{ Excursion measure }=\int_U n(df)(1-e^{-\lambda\zeta(f)}).

We have therefore linked the exponential mark process on excursion measure with the Laplace transform of the entrance law. So in particular:

\frac{\lambda n_\lambda(A)}{\lambda n_\lambda(1)}=\mathbb{P}(\text{first mark when in }A)=\int_0^\infty \lambda e^{-\lambda t}P_t(0,A)dt=\lambda R_\lambda 1_A(0).

The resolvent is relatively easy to calculate explicitly, and so we can find the Laplace transform n_\lambda(A). From this it is generally possible to invert the transform to find the entrance law n_t.

References

A Guided Tour Through Excursions – L. C. G. Rogers.

This pair of posts is very much a paraphrase of chapters 3 and 4 of this excellent text. The original can be found here (possibly not open access?)