4
$\begingroup$

Let $X_t$ be a Brownian motion or a Brownian Bridge on a (\edit: compact) Riemannian manifold. Let $T>0$ be given.

The question is: Does there exists a constant $C>0$ such that for all partitions $0 = \tau_0 < \tau_1 < \dots < \tau_N \leq T$, we have $$ \mathbb{E}\left[ \exp \left(\sum_{j=1}^N d(X_{\tau_{j-1}}, X_{\tau_j})^2 \right) \right] \leq C~~~~~~?$$

I can prove so far that for all $p \in [1, \infty)$, there exists a constant $C_p$ such that $$\mathbb{E}\left[ \left(\sum_{j=1}^N d(X_{\tau_{j-1}}, X_{\tau_j})^2 \right)^p \right] \leq C_p,$$ but I cannot control the constants $C_p$ so that the "brute force proof" using the exponential series fails.

$\endgroup$
4
  • $\begingroup$ Do you assume bounded curvature? If not, I find it surprising how you get any a priori tail bounds on $d(X_0, X_1)$ at all, simply because in regions of arbitrarily largely negative curvature one can have arbitrarily strong drift of the distance, hence arbitrarily heavy tails. And conversely, bounded curvature implies that $d(X_0, X_t)$ differs from its Euclidean counterpart by at most $ct$, where $c$ depends on the curvature bound. I haven't checked the details though, so I might be saying something stupid. $\endgroup$ Commented Oct 13, 2015 at 21:00
  • $\begingroup$ I forgot to mention, yes you may assume bounded curvature; in the setup I am interested in, the manifold is compact. $\endgroup$ Commented Oct 13, 2015 at 21:08
  • $\begingroup$ Do you have a reference for this deviation property? $\endgroup$ Commented Oct 13, 2015 at 21:09
  • $\begingroup$ No, and I'm probably wrong about that on short scales, but what I had in mind is that there are comparison theorems like Theorem 1.1 here: cvgmt.sns.it/media/doc/paper/2113/HessBV.pdf, which imply that if you have a Ricci curvature bound from below then the distance is stochastically dominated by that for the hyperbolic space. The same can be done for each increment conditionally on the past, so basically you only need to check your hypothesis on the hyperbolic space, for which you can calculate everything explicitly. $\endgroup$ Commented Oct 13, 2015 at 21:23

1 Answer 1

2
$\begingroup$

Yes, there is a bound like that for $T \le \mathrm{const}$. I'll do the case of Brownian motion, since the Brownian bridge reduces to it.

The proof consists of two stages: proving the bound in hyperbolic space and reducing the general case to it.

In order to redue the general case to the constant curvature case we can use the comparison theorem for the Laplacian of the distance function (Theorem 1.1 in cvgmt.sns.it/media/doc/paper/2113/HessBV.pdf). Namely:

Theorem: Fix a point $p$ and take the distance function $d_p(\cdot) := d(p, \cdot)$. Then a curvature bound $$\operatorname{Ric} \ge (n-1) K$$ for some constant $K$ implies a bound $$\Delta d_p(r, \theta) \le \Delta^K d^K(r)$$ outside of the cut locus. Here $\Delta^K$ and $d^K$ refers to the Laplacian and distance function of the simply connected constant curvature $K$ space.

For our purposes we won't need to touch the cut locus, so this most naive version suffices.

Recall that the distance process $d_p(X_t)$ (resp. its constant curvature counterpart $d^K(X_t^K)$) is a semimartingale with drift $\frac{1}{2} \Delta d_p(X_t)$ (resp. $\Delta^K d^K(X_t^K)$) and quadratic variation $t$, so they can be represented as solutions to SDEs

$$d (d_p(X_t)) = \frac{1}{2} \Delta d_p(X_t) dt + dB_t$$ $$d (d^K(X_t^K)) = \frac{1}{2} \Delta^K d^K(X_t^K) dt + dB_t$$

where $B$ is a standard $1$-dimensional Brownian motion. By the Laplacian comparison theorem we have an inequality between the drift terms, and I intentionally used the same driving noise $dB$ in both SDEs. This way the processes become coupled so that

$d_p(X_0) \le d_p^K(X_0^K)$ implies $d_p(X_t) \le d_p^K(X_t^K)$ for all $t$ at least until $X_t$ meets the cut locus.

(see e.g. the proof of Theorem 20.5 in Kallenberg's "Foundations of modern probability")

So let's start $X_t$ at the point $p$ and consider the stopped process $X_{t \wedge \theta}$, $\theta := \inf\{t : d_p(X_t) = R\}$, where $R$ is the injectivity radius of our manifold. Similarly, take $\theta^K := \inf\{t : d^K(X_t^K) = R\}$. From the above reasoning, $d_p(X_{t \wedge \theta}) \le d^K(X^K_{t \wedge \theta^K})$ for all $t$, so

$$\mathsf{E} \exp (d(X_0, X_{t\wedge\theta}))^2 \le \mathsf{E} \exp (d^K(X_0^K, X^K_{t\wedge\theta^K}))^2,$$ $$\theta \ge \theta^K$$

In order to get rid of this $\theta$ stopping, just use the trivial inequality $$\mathsf{E} \exp (d(p, X_t))^2 \le \mathsf{E} \exp (d(p, X_{t \wedge \theta}))^2 + \exp D^2 \cdot \mathsf{P} \{\theta \le t\},$$ where $D$ is the diameter of our manifold.

Finally, this gives:

$$\mathsf{E} \exp (d(p, X_t))^2 \le \mathsf{E} \exp(d^K(X_t^K))^2 + \exp D^2 \cdot \mathsf{P}\{\theta_K \le t\}$$

Denote the right-hand side by $F(t)$.

Now note that whenever we have a sequence of times $0 = \tau_0 < \dots \le \tau_N$ we can use the Markov property of the BM to get the same bound conditionally on $X_{\tau_{N-1}}$, then on $X_{\tau_{N-2}}$, etc.:

$$\mathsf{E} \prod_{k < N} \exp (d(X_{\tau_k}, X_{\tau_{k+1}}))^2 \le \dots \le F(\tau_1 - \tau_0) \dots F(\tau_N - \tau_{N-1})$$

Now, in order to deal with the constant curvature case one can use the same approach as above: use a bound like $\Delta^K d^K(r) \le \frac{n-1}{r} + O(1)$ in order to dominate $d^K(X_t^K)$ by a Bessel($n$) process with constant drift, to reduce everything to the trivial case of Brownian motion in $\mathbb{R}^n$.

Similarly, the Brownian bridge case reduces to Brownian motion. Indeed, Brownian bridge on a time interval $[0,T/2]$ is just a Brownian motion with bounded drift, and $[T/2, T]$ it's a time-reversed Brownian motion with bounded drift.

$\endgroup$
6
  • $\begingroup$ BTW, correct me if I'm wrong, but it seems like at the cut locus the singular part of the drift of $d_p(X_t)$ is negative anyway, so whatever happens there, it happens in the right direction. So maybe we don't need to introduce this ugly $\theta$ stopping after all. $\endgroup$ Commented Oct 14, 2015 at 0:33
  • $\begingroup$ You are right. I found this in Hsu's book "Stochastic Analysis on Manifolds" (Thm. 3.5.1) that to make your formula hold for all times, one has to subtract a non-decreasing process, which only increases at the cut locus. So one can in fact leave out the cutoff term. $\endgroup$ Commented Oct 14, 2015 at 10:48
  • $\begingroup$ However, could you elaborate on how to deal with Brownian motion? Because unfortunately, I am not really sure how to deal with the drift term... $\endgroup$ Commented Oct 14, 2015 at 10:48
  • $\begingroup$ And another question: How does this coupling of the processes work? The number 20.5 in my issue of Kallenberg is a Lemma, and its proof has nothing to do with this, so far as I can tell. Thank you very much in advance! $\endgroup$ Commented Oct 14, 2015 at 16:05
  • $\begingroup$ I used the first edition of Kallenberg. In the second edition it's Theorem 23.5. $\endgroup$ Commented Oct 14, 2015 at 20:03

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .