# Borel-TIS inequality

## Theorem

Let $T$ be a topological space.

Let $\sequence {f_t}_{t \mathop \in T}$ be a centred (i.e. mean zero) Gaussian process on $T$, such that:

- $\norm f_T := \sup_{t \mathop \in T} \size {f_t}$

is almost surely finite.

Let:

- $\sigma_T^2 := \sup_{t \mathop \in T} \operatorname{E} \size {f_t}^2$

Then $\map {\operatorname {E} } {\norm f_T}$ and $\sigma_T$ are both finite, and, for each $u > 0$:

- $\map {\operatorname {P} } {\norm f_T > \map {\operatorname {E} } {\norm f_T} + u} \le \map \exp {\dfrac {- u^2} {2 \sigma_T^2} }$