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Theory of Statistics - George Mason University

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0.0.2 Sets and Spaces<br />

0.0 Some Basic Mathematical Concepts 615<br />

In Section 0.0.9 beginning on page 670, we discussed some basics <strong>of</strong> sets and<br />

operations on sets. Now we consider mathematical structures that are built on<br />

sets together with other objects or methods such as an operation on the given<br />

set or on subsets <strong>of</strong> the set. We <strong>of</strong>ten refer to these structures as “spaces”.<br />

0.0.2.1 Spaces<br />

In any application it is generally useful to define some “universe <strong>of</strong> discourse”<br />

that is the set <strong>of</strong> all elements that will be considered in a given problem. Given<br />

a universe or universal set, which we <strong>of</strong>ten denote by the special symbol Ω<br />

(note the font), we then define various mathematical structures on Ω. These<br />

structures, or “spaces”, are formed by specifying certain types <strong>of</strong> collections<br />

<strong>of</strong> subsets <strong>of</strong> Ω and/or by defining operations on the elements <strong>of</strong> Ω or on the<br />

subsets in the special collection <strong>of</strong> subsets. In probability and statistics, we<br />

will call the universal set the sample space.<br />

Some <strong>of</strong> the general structures that we will find useful are topological<br />

spaces, which are defined in terms <strong>of</strong> the type <strong>of</strong> collection <strong>of</strong> subsets <strong>of</strong> the<br />

universal set, and metric spaces and linear spaces, which are defined in terms<br />

<strong>of</strong> operations on elements <strong>of</strong> the universal set. We will discuss these below,<br />

and then in Section 0.0.5, we will discuss some properties <strong>of</strong> the special spaces<br />

in which the universal set is the set <strong>of</strong> real numbers. In Section 0.1, we will<br />

discuss various types <strong>of</strong> collections <strong>of</strong> subsets <strong>of</strong> the universal set, and then<br />

for a particular type <strong>of</strong> collection, called a σ-field, we will discuss a special<br />

type <strong>of</strong> space, called a measurable space, and then, with the addition <strong>of</strong> a<br />

real-valued set function, we will define a measure space. A particular type <strong>of</strong><br />

measure space is a probability space.<br />

0.0.2.2 Topologies<br />

One <strong>of</strong> the simplest structures based on the nonempty universal set Ω is a<br />

topological space or a topology, which is formed by any collection T <strong>of</strong> subsets<br />

<strong>of</strong> Ω with the following properties:<br />

(t1) ∅, Ω ∈ T , and<br />

(t2) A, B ∈ T ⇒ A ∩ B ∈ T , and<br />

(t3) A ⊆ T ⇒ ∪{A : A ∈ A} ∈ T .<br />

We denote a topological space by a double <strong>of</strong> the form (Ω, T ).<br />

We may use the term “topology” to denote either the space or the collection<br />

<strong>of</strong> subsets that defines it.<br />

Properties <strong>of</strong> Ω that can be expressed in terms <strong>of</strong> a topology are called its<br />

topological properties. Without imposing any additional structure on a topological<br />

space, we can define several useful concepts, but because the collection<br />

<strong>of</strong> subsets that define a topology is arbitrary, many terms that relate to a<br />

<strong>Theory</strong> <strong>of</strong> <strong>Statistics</strong> c○2000–2013 James E. Gentle

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