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Theory of Statistics - George Mason University

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670 0 Statistical Mathematics<br />

(for the first one), choose any x ∈ A and show x ∈ B. The same technique is<br />

used to show that two collections <strong>of</strong> sets, for example, two σ-fields, are equal.<br />

To show that a sequence converges, use partial sums and an ɛ bound.<br />

To show that a series converges, show that the sequence is a Cauchy sequence.<br />

The standard procedures may not always work, but try them first. In<br />

the next section, I describe several facts that are <strong>of</strong>ten used in mathematical<br />

pro<strong>of</strong>s.<br />

0.0.9 Useful Mathematical Tools and Operations<br />

In deriving results or in proving theorems, there are a number <strong>of</strong> operations<br />

that occur over and over. It is useful to list some <strong>of</strong> these operations so that<br />

they will more naturally come to mind when they are needed. The following<br />

subsections list mathematical operations that should be in fast memory. None<br />

<strong>of</strong> these should be new to the reader. In some cases, we mention a specific<br />

operation such as completing the square; in other cases, we mention a specific<br />

formula such as De Morgan’s laws or the inclusion-exclusion formula.<br />

0.0.9.1 Working with Abstract Sets<br />

Two <strong>of</strong> the most useful relations are De Morgan’s laws, equations (0.0.2)<br />

and (0.0.3), and their extensions to countable unions and intersections.<br />

The inclusion-exclusion formula, equation (0.0.8), is particularly useful in<br />

dealing with collections <strong>of</strong> subsets <strong>of</strong> a sample space.<br />

For a general sequence <strong>of</strong> sets {An}, the disjoint sequence (0.0.6) {Dn} on<br />

page 612 that partitions their union is <strong>of</strong>ten useful.<br />

If the sequence {An} is increasing, that is, A1 ⊆ A2 ⊆ . . ., the intersection<br />

is trivial, but the union ∪ ∞ n=1An may be <strong>of</strong> interest. In that case, the disjoint<br />

sequence (0.0.7) Dn = An+1 − An may be useful. Conversely, if the sequence<br />

{An} is decreasing, the union is trivial, but the intersection may be <strong>of</strong> interest.<br />

In that case, De Morgan’s laws may be used to change the decreasing sequence<br />

into an increasing one.<br />

0.0.9.2 Working with Real Sets<br />

There are many useful properties <strong>of</strong> real numbers that simplify operations<br />

on them. Recognizing common sequences <strong>of</strong> reals as discussed beginning on<br />

page 645 or sequences <strong>of</strong> real intervals discussed beginning on page 639 will<br />

aid in solving many problems in mathematics. The sequences <strong>of</strong> intervals<br />

<br />

Oi = a − 1<br />

<br />

1<br />

, b + (0.0.73)<br />

i i<br />

and<br />

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

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