# 3.4.3. Uniform Law of Convex Hulls

$\newcommand{\argmin}{\mathop{\mathrm{argmin}}\limits}$ $\newcommand{\argmax}{\mathop{\mathrm{argmax}}\limits}$

The last example of the ULLN that van de Geer (2000) present regards the convex hull of a function class. Take this subsection as a cherry on top.

## Convex hull

The convex hull $\text{conv}(\mathcal H)$ of a function class $\mathcal H$ is defined as the set of all finite convex combinations of functions in $\mathcal H.$ That is,

$\text{conv}(\mathcal H) = \left\{ \sum_{j=1}^r \theta_jh_j:~ h_j \in \mathcal H,~ \theta_j \ge 0,~ \sum_{j=1}^r \theta_j = 1,~, 1\le j\le r,~ r\in\mathbb N \right\}.$

## ULLN of convex hulls

In fact, deriving the uniform law of convex hulls does not utilize entropy inequality at all if the corresponding function class satisfies the ULLN.

Suppose that $\mathcal H \subset L^1(P)$ satisfies the ULLN. Then $\text{conv}(\mathcal H)$ satisfies the ULLN as well.

Given $r\in\mathbb N,$ arbitrarily take $\theta_j \ge 0,$ $h_j \in \mathcal H$ for $1\le j\le r.$ Then \begin{aligned} &\left| \int \sum_{j=1}^r \theta_j h_j d(P_n-P) \right| \\ &= \left| \sum_{j=1}^r \theta_j \int h_jd(P_n-P) \right| \\ &\le \cancel{\left(\sum_{j=1}^r \theta_j\right)}^1 \max_{1\le j\le r}\left|\int h_jd(P_n-P)\right|. \end{aligned} Since $\mathcal H$ satisfies the ULLN, the right-hand side approaches zero as $n$ goes to infinity.

## Convex hull inside the unit ball in $L^2$

For a more sophisticated argument, we do need help of entropy inequality.

Let $Q$ be a probability measure on $(\mathcal X, \mathcal A)$ and $\mathcal H \subset B_{L^2(Q)}(0, 1)$ where $B_{L^2(Q)}(0, 1)=\{h:~\int h^2dQ\le 1\}.$ Then $$N_{2,Q}(\delta, \mathcal H) \le c\frac1{\delta^d},~ \forall \delta>0 \\ \implies \exists A(c,d)=A \text{: }~ H_{2,Q}(\delta, \text{conv}(\mathcal H)) \le A \frac{1}{\delta^{2d/(2+d)}},~ \forall \delta>0$$

The proof is in Ball and Pajor (1990).

### Example: A function class as a convex hull

$\mathcal G = \{g:\mathbb R\to[0,1] \text{ is increasing}\}.$

Notice that $\mathcal G \sub B_{L^2(Q)}(0,1)$ and is in fact for $\mathcal H = {\mathbf 1_{[y,\infty)}:~ y\in\mathbb R},$

$\mathcal G = \overline{\text{conv}}(\mathcal H).$

i.e. is a closure of the convex hull of $\mathcal H.$ Given $\delta>0,$ let

$\mathcal Y_\delta=\{y_0, y_1,\cdots, y_{N-1}, y_N\}, \\ y_0 = Q^{-1}(\delta^2),~ y_N=Q^{-1}(\delta^2), \\ Q([y_{i-1},y_{i})) \le \delta^2,~ 1\le i\le N$

where $Q^{-1}(p)$ is the $(p\times100)$-percentile number of the probability measure $Q.$ Then it is clear that $\{\mathbf 1_ {[y,\infty)}:~ y\in\mathcal Y_ \delta\}$ is a $\delta$-covering. Thus for some constant $c,$ we have

$N_{2,Q}(\delta, \mathcal H) \le c\frac1{\delta^2}, ~\forall \delta >0$

Hence by theorem 3.14, for some constant $A$ depending only on $c,$ we get the entropy bound

$H_{2,Q}(\delta,\mathcal G) \le A\frac1\delta,~ \forall \delta>0$

since the closure does not affect the covering number. Compare this to the result (1) of entropy inequalities 1.

References

• van de Geer. 2000. Empirical Processes in M-estimation. Cambridge University Press.
• Theory of Statistics II (Fall, 2020) @ Seoul National University, Republic of Korea (instructor: Prof. Jaeyong Lee).