# Zulip Chat Archive

## Stream: new members

### Topic: The σ-algebra of cylinder sets of Brownian trajectories

#### Lars Ericson (Dec 21 2020 at 04:18):

We have a set Ω of all continuous real functions from `nnreal`

to ℝ:

```
@[derive [topological_space, has_coe_to_fun]]
def Brownian_event_space := C(nnreal, ℝ)
local notation `Ω` := Brownian_event_space
noncomputable instance : measurable_space Ω := borel _
instance : borel_space Ω := ⟨rfl⟩
```

We define the **cylinder sets** `C T I`

on Ω as describedf by Zeev Schuss:

Screenshot-from-2020-12-20-23-12-42.png

#### Lars Ericson (Dec 21 2020 at 04:19):

so

```
def interval (n : ℕ ) (I : (fin n) → ℝ × ℝ) (k: (fin n)) := set.Ioo (I k).1 (I k).2
def cylinder_set (n : ℕ) (t : fin n → nnreal)
(I : (fin n) → ℝ × ℝ ) :=
{ω : Brownian_event_space | ∀ k : fin n, (ω (t k) ∈ (interval n I k))}
#check cylinder_set -- : Π (n : ℕ), (fin n → nnreal) → (fin n → ℝ × ℝ) → set Ω
```

#### Lars Ericson (Dec 21 2020 at 04:20):

The space of Brownian Events is the σ-algebra of the cylinder sets, so:

Screenshot-from-2020-12-20-23-13-28.png

#### Lars Ericson (Dec 21 2020 at 04:31):

The enumeration here is over all possible time grids `T`

of size `n`

and all possible real intervals of size `n`

. So countable for the size and then doubly uncountable for the, for each size, to enumerate all possible cylinder bounds. The sample space is all members of `C(nnreal, ℝ)`

.

So for probability space, instead of a setting `(Ω, A, P)`

, where the event space is `A=B(Ω)`

, i.e. the Borel sets of Ω, here we've got the σ-algebra generated by the countable/double uncountable family of cylinder sets of Ω.

So I guess I can do that with `measurable_space.generate_from`

on the cylinder sets, if I can figure out how to enumerate all the `t`

and `I`

.

No specific question here....just if this rings a bell and you have any thoughts, please say a few words.

All I'm trying to do here is give the construction of the probability space for Brownian motion, in Lean.

This is not much discussed but is pretty explicitly described in the Zeev Schuss book linked above. I've Googled a lot and most discussions don't quite come right out and build the space, but Zeev does. I like descriptions like that that are more constructive and hands-on. In this case it seems a bit tricky though because of the uncountables.

#### Reid Barton (Dec 21 2020 at 16:41):

It looks like the Brownian events are intended to make up the sigma-algebra on `Ω`

, so you don't want the Borel one--you can just delete the last two lines of your very first code block, and you probably don't use the topological space structure at all.

#### Reid Barton (Dec 21 2020 at 16:41):

(It's not clear to me whether in the end you do get the same sigma-algebra after all.)

Last updated: May 06 2021 at 22:13 UTC