## Sub-Gaussian property for the Beta distribution (part 1)

With my friend Olivier Marchal (mathematician, not filmmaker, nor the cop), we have just arXived a note on the sub-Gaussianity of the Beta and Dirichlet distributions.

The notion, introduced by Jean-Pierre Kahane, is as follows:

A random variable with finite mean is sub-Gaussian if there is a positive number such that:

Such a constant is called a proxy variance, and we say that is -sub-Gaussian. If is sub-Gaussian, one is usually interested in the optimal proxy variance:

Note that the variance always gives a lower bound on the optimal proxy variance: . In particular, when , is said to be

strictlysub-Gaussian.

The sub-Gaussian property is closely related to the tails of the distribution. Intuitively, being sub-Gaussian amounts to having tails lighter than a Gaussian. This is actually a characterization of the property. Let . Then:

That equivalence clearly implies exponential upper bounds for the tails of the distribution since a Gaussian satisfies

That can also be seen directly: for a -sub-Gaussian variable ,

The polynomial function is minimized on at , for which we obtain

.

In that sense, the sub-Gaussian property of any compactly supported random variable comes for free since in that case the tails are obviously lighter than those of a Gaussian. A simple general proxy variance is given by Hoeffding’s lemma. Let be supported on with . Then for any ,

so is -sub-Gaussian.

Back to the Beta where , this shows the Beta is -sub-Gaussian. The question of finding the optimal proxy variance is a more challenging issue. In addition to characterizing the optimal proxy variance of the Beta distribution in the note, we provide the simple upper bound . It matches with Hoeffding’s bound for the extremal case , , where the Beta random variable concentrates on the two-point set (and when Hoeffding’s bound is tight).

In getting the bound , we prove a recent conjecture made by Sam Elder in the context of Bayesian adaptive data analysis. I’ll say more about getting the optimal proxy variance in a next post soon.

Cheers!

Julyan

Sub-Gaussian property for the Beta distribution (part 2) | Statisfactionsaid, on 20 December 2017 at 15:39[…] a follow-up on my previous post on the sub-Gaussian property for the Beta distribution [1], I’ll give here a visual […]

Sub-Gaussian property for the Beta distribution (part 2) – Cloud Data Architectsaid, on 20 December 2017 at 17:00[…] a follow-up on my previous post on the sub-Gaussian property for the Beta distribution [1], I’ll give here a visual illustration […]

Sub-Gaussian property for the Beta distribution (part 2) - bivasaid, on 20 December 2017 at 17:09[…] a follow-up on my previous post on the sub-Gaussian property for the Beta distribution [1], I’ll give here a visual illustration […]

Sub-Gaussian property for the Beta distribution (part 2) – Mubashir Qasimsaid, on 20 December 2017 at 21:17[…] a follow-up on my previous post on the sub-Gaussian property for the Beta distribution [1], I’ll give here a visual […]

Sub-Gaussian property for the Beta distribution (part 3, final) | Statisfactionsaid, on 26 December 2017 at 15:12[…] this third and last post about the Sub-Gaussian property for the Beta distribution [1] (post 1 and post 2), I would like to show the interplay with the Bernoulli distribution as well as some […]

Sub-Gaussian property for the Beta distribution (part 3, final) – Cloud Data Architectsaid, on 26 December 2017 at 22:00[…] this third and last post about the Sub-Gaussian property for the Beta distribution [1] (post 1 and post 2), I would like to show the interplay with the Bernoulli distribution as well as some […]

Sub-Gaussian property for the Beta distribution (part 3, final) - bivasaid, on 26 December 2017 at 22:11[…] this third and last post about the Sub-Gaussian property for the Beta distribution [1] (post 1 and post 2), I would like to show the interplay with the Bernoulli distribution as well as some […]

Sub-Gaussian property for the Beta distribution (part 3, final) – Mubashir Qasimsaid, on 27 December 2017 at 01:20Maguss cheatssaid, on 12 March 2018 at 03:39I was recommended this blog via my cousin. I am not certain whether or not this submit is written by way of

him as nobody else recognize such distinct approximately my

problem. You’re amazing! Thanks!