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Theory of Statistics - George Mason University

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708 0 Statistical Mathematics<br />

Notice what is not said above (where we use the word “finite”).<br />

A very important type <strong>of</strong> set is an interval in IR. Intervals are the basis for<br />

building important structures on IR. All intervals are Borel sets. We discussed<br />

properties <strong>of</strong> real intervals and, in particular, sequences on real intervals beginning<br />

on page 639.<br />

0.1.4.1 The Borel σ-Field on the Reals<br />

On page 691, we have defined a Borel σ-field for a topological space as the<br />

σ-field generated by the topology, that is, by the collection <strong>of</strong> open sets that<br />

define the topology. In a metric space, such as IR, we define open sets in terms<br />

<strong>of</strong> the metric, and then we define a Borel σ-field as before in terms <strong>of</strong> those<br />

open sets. The most interesting topological space is the set <strong>of</strong> reals together<br />

with the class <strong>of</strong> open intervals, (IR, C).<br />

Definition 0.1.25 (Borel σ-field)<br />

Let C be the collection <strong>of</strong> all open intervals in IR. The σ-field σ(C) is called<br />

the Borel σ-field over IR, and is denoted by B(IR).<br />

We <strong>of</strong>ten call this Borel σ-field over IR just the Borel field, and denote it<br />

just by B.<br />

0.1.4.2 Borel Sets<br />

Any set in B is called a Borel set. Such sets are said to be “Borel measurable”,<br />

from the fact that they are λ-measurable, for the Lebesgue measure λ in<br />

equation (0.1.20).<br />

Example 0.1.4 (Borel-measurable sets)<br />

The following are all Borel-measurable sets.<br />

1. IR<br />

2. ∅<br />

3. any countable set; in particular, any finite set, ZZ, ZZ+ (the natural numbers),<br />

and the set <strong>of</strong> all rational numbers<br />

4. hence, from the foregoing, the set <strong>of</strong> all irrational numbers (which is uncountable)<br />

5. any interval, open, closed, or neither<br />

6. the Cantor set<br />

The Cantor set is ∩ ∞ i=1Ci, where<br />

C1 = [0, 1/3]∪[2/3,1], C2 = [0, 1/9]∪[2/9, 1/3]∪[2/3,7/9]∪[8/9,1], . . .,<br />

We see that each <strong>of</strong> these is Borel, and hence, so is the intersection. A<br />

Cantor set has interesting properties; for example, its cardinality is the<br />

same as that <strong>of</strong> the interval [0, 1], yet it is nowhere dense in [0, 1]. (The<br />

particular set described here is the Cantor ternary set; other similar sets<br />

are also called Cantor sets.)<br />

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

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