## Monte Carlo with determinantal processes [reply from the authors]

Posted in Books, Statistics with tags , , , , , , , , , , , , , , on September 22, 2016 by xi'an

[Rémi Bardenet and Adrien Hardy have written a reply to my comments of today on their paper, which is more readable as a post than as comments, so here it is. I appreciate the intention, as well as the perfect editing of the reply, suited for a direct posting!]

Thanks for your comments, Xian. As a foreword, a few people we met also had the intuition that DPPs would be relevant for Monte Carlo, but no result so far was backing this claim. As it turns out, we had to work hard to prove a CLT for importance-reweighted DPPs, using some deep recent results on orthogonal polynomials. We are currently working on turning this probabilistic result into practical algorithms. For instance, efficient sampling of DPPs is indeed an important open question, to which most of your comments refer. Although this question is out of the scope of our paper, note however that our results do not depend on how you sample. Efficient sampling of DPPs, along with other natural computational questions, is actually the crux of an ANR grant we just got, so hopefully in a few years we can write a more detailed answer on this blog! We now answer some of your other points.

“one has to examine the conditions for the result to operate, from the support being within the unit hypercube,”
Any compactly supported measure would do, using dilations, for instance. Note that we don’t assume the support is the whole hypercube.

“to the existence of N orthogonal polynomials wrt the dominating measure, not discussed here”
As explained in Section 2.1.2, it is enough that the reference measure charges some open set of the hypercube, which is for instance the case if it has a density with respect to the Lebesgue measure.

“to the lack of relation between the point process and the integrand,”
Actually, our method depends heavily on the target measure μ. Unlike vanilla QMC, the repulsiveness between the quadrature nodes is tailored to the integration problem.

“changing N requires a new simulation of the entire vector unless I missed the point.”
You’re absolutely right. This is a well-known open issue in probability, see the discussion on Terence Tao’s blog.

“This requires figuring out the upper bounds on the acceptance ratios, a “problem-dependent” request that may prove impossible to implement”
We agree that in general this isn’t trivial. However, good bounds are available for all Jacobi polynomials, see Section 3.

“Even without this stumbling block, generating the N-sized sample for dimension d=N (why d=N, I wonder?)”
This is a misunderstanding: we do not say that d=N in any sense. We only say that sampling from a DPP using the algorithm of [Hough et al] requires the same number of operations as orthonormalizing N vectors of dimension N, hence the cubic cost.

1. “how does it relate to quasi-Monte Carlo?”
So far, the connection to QMC is only intuitive: both rely on well-spaced nodes, but using different mathematical tools.

2. “the marginals of the N-th order determinantal process are far from uniform (see Fig. 1), and seemingly concentrated on the boundaries”
This phenomenon is due to orthogonal polynomials. We are investigating more general constructions that give more flexibility.

3. “Is the variance of the resulting estimator (2.11) always finite?”
Yes. For instance, this follows from the inequality below (5.56) since ƒ(x)/K(x,x) is Lipschitz.

4. and 5. We are investigating concentration inequalities to answer these points.

6. “probabilistic numerics produce an epistemic assessment of uncertainty, contrary to the current proposal.”
A partial answer may be our Remark 2.12. You can interpret DPPs as putting a Gaussian process prior over ƒ and sequentially sampling from the posterior variance of the GP.

## Monte Carlo with determinantal processes

Posted in Books, Statistics with tags , , , , , , , , on September 21, 2016 by xi'an

Rémi Bardenet and Adrien Hardy have arXived this paper a few months ago but I was a bit daunted by the sheer size of the paper, until I found the perfect opportunity last week..! The approach relates to probabilistic numerics as well as Monte Carlo, in that it can be seen as a stochastic version of Gaussian quadrature. The authors mention in the early pages a striking and recent result by Delyon and Portier that using an importance weight where the sampling density is replaced with the leave-one-out kernel estimate produces faster convergence than the regular Monte Carlo √n! Which reminds me of quasi-Monte Carlo of course, discussed in the following section (§1.3), with the interesting [and new to me] comment that the theoretical rate (and thus the improvement) does not occur until the sample size N is exponential in the dimension. Bardenet and Hardy achieve similar super-efficient convergence by mixing quadrature with repulsive simulation. For almost every integrable function.

The fact that determinantal point processes (on the unit hypercube) and Gaussian quadrature methods are connected is not that surprising once one considers that such processes are associated with densities made of determinants, which matrices are kernel-based, K(x,y), with K expressed as a sum of orthogonal polynomials. An N-th order determinantal process in dimension d satisfies a generalised Central Limit Theorem in that the speed of convergence is

$\sqrt{N}^{(d-1)/d}$

which means faster than √N…  This is more surprising, of course, even though one has to examine the conditions Continue reading