# Supremum of a stochastic process

Let $(x_1,...,x_N)$ be points in $R^d$, and $\sigma=(\sigma_1,...,\sigma_N)$ are i.i.d. Rademacher variables (+1 or -1 with probability 0.5 each). (Or alternatively, $\sigma$ could be a standard Gaussian vector.) Also let $z$ in an infinite subset of $R^d$, and $\|\cdot\|_2$ will denote the Euclidean norm. I am looking for a good upper estimate of:

$E_{\sigma} \sup_z \| \sum_{n=1}^N \sigma_n \frac{x_n-z}{\|x_n-z\|_2} \|_2$.

So far I only got the upper bound of $N$ (by triangle inequality), and the lower bound $\sqrt{N}$ by Jensen inequality, and I would like to know if there is a way to improve on the upper estimate or the lower estimate?

If not, are there conditions (on the domain of $z$) under which the upper bound would reduce to ${\mathcal O}(\sqrt{N})$?

Edit: The points $x_1,...,x_N$ are assumed to be in general position (in a ball of radius $R$).

Edit2: The domain of values of $z$ is an infinite set in $R^d$. Can be assumed to be bounded.

Edit3: The domain of $z$ is allowed to depend on $N$, $R$ (or on other assumed characteristics of the point set), but not on the individual points.

Edit4: I also want to avoid dependence on $d$.

• So is the domain of $z$ allowed to depend on the $x_i$? – Bjørn Kjos-Hanssen Apr 22 '17 at 6:02
• Preferably not, but it's okay to let it depend on inter-point distances. – axk Apr 22 '17 at 6:07

First, if the subset $Z$ of $R^d$ of allowed values for $z$ is a singleton, then writing $e_n=(x_n-z)/\|x_n-z\|_2=(e_{n,j})_{j=1}^d$, we have $$E_\sigma \left\|\sum\sigma_ne_n\right\|_2=E_\sigma\sqrt{\sum_j \left(\sum_n\sigma_ne_{n,j}\right)^2}\le\sqrt{E_\sigma\sum_j\left(\sum_n\sigma_ne_{n,j}\right)^2}$$ $$%=\sqrt{\sum_jE_\sigma\left[\left(\sum_n\sigma_ne_{n,j}\right)^2\right]} =\sqrt{\sum_j\mathrm{Var}_\sigma\left[\sum_n\sigma_ne_{n,j}\right]} =\sqrt{\sum_j\sum_n\mathrm{Var}_\sigma\left[\sigma_ne_{n,j}\right]}$$ $$=\sqrt{\sum_j\sum_ne_{n,j}^2}=\sqrt{\sum_n \|e_n\|_2^2}=\sqrt{N}.$$ Second, if $Z$ is allowed to depend on $N$, then let $Z$ be of very small diameter $\delta>0$, relative to
• the distances $\|x_i-x_j\|_2$,
• $N$, and
• the modulus of continuity of the function $z\mapsto \left\| \sum \sigma_n e_n\right\|_2$,
then we should be able to get for a fixed $z_0\in Z$, and for all $\sigma$, $$\sup_{z\in Z}\left\|\sum\sigma_ne_n\right\|_2 \le \left\|\sum\sigma_ne_n\text{ (for z_0)}\right\|_2 + \epsilon$$ and hence $$E_\sigma \left(\sup_z\left\|\sum\sigma_ne_n\right\|_2\right) \le E_\sigma\left( \left\|\sum\sigma_ne_n\text{ (for z_0)}\right\|_2 + \epsilon\right) = \mathcal O(\sqrt{N}).$$