σ-FIELD (cont.): Mathematically, we will require this collection to be a σ-field, a term that we define next.
The term “event” is to be understood as follows. Once the experiment is concluded, the realized outcome ω either belongs to A, in which case we say that the event A has occurred, or it doesn’t, in which case we say that the event did not occur.
Exercise:
(a) Let F be an σ-field. Prove that if A, B ∈F, then A∩B ∈F. More generally, given a count ably infinite sequence of events Ai ∈F, prove that ∩∞i=1Ai ∈F.
(b) Prove that property (a) of σ-fields (that is, Ø ∈F) can be derived from properties
(b) and (c), assuming that the σ-field F is non-empty. The following are some examples of σ-fields
Proposition 1:
Let S be an index set (possibly infinite, or even uncountable), and suppose that for every s we have a σ-field Fs of subsets of the same sample space. Let F = ∩s∈SFs, i.e., a set A belongs to F if and only if A ∈Fs for every s ∈S. Then F is an σ-field.
Proof:
We need to verify that F has the three required properties. Since each Fs is a σ-field, we have Ø ∈Fs, for every s, which implies that Ø ∈F. To establish the second property, suppose that A ∈F. Then, a ∈Fs, for every s.
Since each s is a σ-field, we have Ac ∈Fs, for every s. Therefore, Ac ∈F, as desired. Finally, to establish the third property, consider a sequence {Ai} of elements of F. In particular, for a given s ∈S, every set Ai belongs to Fs. Since Fs is a σ-field, it follows that ∪∞. Since this is true for every s ∈S, i=1
Ai ∈Fs it follows that ∪∞i=1 Ai ∈Fs. This verifies the third property and establishes that F is indeed a σ-field.
Suppose now that we start with a collection C of subsets of Ω, which is not necessarily a σ-field. However, for technical reasons, we may wish the σ-field to contain no more sets than necessary. This
leads us to define F as the intersection of all σ-fields that contain C. Note that if H is any other σ-field that contains C, then F ⊂H. (This is because F was defined as the intersection of various σ-fields, one of which is H.) In this sense, F is the smallest σ-field containing C. The σ-field F constructed in this manner is called the σ-field generated by C, and is often denoted by σ(C).
Example. Let Ω = [0, 1]. The smallest σ-field that includes every interval [a, b] ⊂[0, 1] is hard to describe explicitly (it nincludesfairly complicated sets), but is still well-defined, by the above discussion. It is called the Borel σ-field, and is denoted by B. A set A ⊂[0, 1] that belongs to this σ-field is called a Borel set.