# How to understand Demartines theorem

Demartines theorem (page 3) explain why norm of a random vector increases with dimension, whereas its variance remains constant:

Let $X \in \mathbb{R}^d$ be a random vector with i.i.d. components: $X_i \sim \mathcal{F}$. Then, $$\text{E} \left[ \|X\|_2 \right] = \sqrt{ad-b} + O \left( \frac{1}{d} \right)$$ and $$\text{Var} \left( \| X \|_2 \right) = b + O \left( \frac{1}{\sqrt{d}} \right)$$ where $a$ and $b$ are constants that do not depend on the dimension.

The theorem proves that the expectation of the euclidean norm of random vectors increases as the square root of the dimension $d$, whereas its variance is constant and independent of the dimension. When the dimension is large, the variance of the norm is very small compared with its expected value.

My questions:

1. What does $O$ stands for?
2. Why does norm increases with the square root of dimension? How can we see that?
3. Why is variance constant? How can we see that?

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• This is a minor variant of the Central Limit Theorem, which asserts the standardized version of $||X||_2^2$ is asymptotically Normal. It requires that $X$ have finite variance. – whuber Jul 1 '14 at 15:28