- #1

Fredrik

Staff Emeritus

Science Advisor

Gold Member

- 10,851

- 413

- Thread starter Fredrik
- Start date

- #1

Fredrik

Staff Emeritus

Science Advisor

Gold Member

- 10,851

- 413

- #2

- 22,089

- 3,293

Yes, in a trivial way. Just define [itex]\mu(S)=0[/itex] for all S. Or take the counting measure. Or a point measure.

A better answer is actually Riesz representation theorem. This says that all regular measures on a locally compact Hausdorff space coincide with positive functionals

[tex]T:C_c(X)\rightarrow \mathbb{R}[/tex]

(with [itex]C_c(X)[/itex] the functions to [itex]\mathbb{R}[/itex] with compact support). Such a functional exist because of the Hahn-Banach theorem.

The same thing can be done with [itex]C_0(X)[/itex] actually. In that case: the measures are the positive functionals, the probability measures are the states and the point measures represent the pure states on X.

In general, the existence of non-trivial measures is not a simple problem and requires set theory (see for example measurable cardinals)

A better answer is actually Riesz representation theorem. This says that all regular measures on a locally compact Hausdorff space coincide with positive functionals

[tex]T:C_c(X)\rightarrow \mathbb{R}[/tex]

(with [itex]C_c(X)[/itex] the functions to [itex]\mathbb{R}[/itex] with compact support). Such a functional exist because of the Hahn-Banach theorem.

The same thing can be done with [itex]C_0(X)[/itex] actually. In that case: the measures are the positive functionals, the probability measures are the states and the point measures represent the pure states on X.

In general, the existence of non-trivial measures is not a simple problem and requires set theory (see for example measurable cardinals)

Last edited:

- #3

lavinia

Science Advisor

Gold Member

- 3,237

- 624

- #4

- 22,089

- 3,293

The first thing to do is to define the word "the trivial one".

This is quite an interesting theory. In general, this is studied in Boolean algebras and the so-called measure algebras.

- #5

Fredrik

Staff Emeritus

Science Advisor

Gold Member

- 10,851

- 413

Excellent answer as always. Thanks. I've been meaning to study the Riesz representation theorem. It will be the first thing I do when I'm done with the basics of integration theory.

Last edited:

- Replies
- 1

- Views
- 3K

- Last Post

- Replies
- 2

- Views
- 16K

- Last Post

- Replies
- 8

- Views
- 10K

- Last Post

- Replies
- 2

- Views
- 4K

- Last Post

- Replies
- 1

- Views
- 4K

- Last Post

- Replies
- 2

- Views
- 6K

- Last Post

- Replies
- 6

- Views
- 3K

- Last Post

- Replies
- 1

- Views
- 3K

- Last Post

- Replies
- 2

- Views
- 3K

- Last Post

- Replies
- 25

- Views
- 4K