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Theory of Statistics - George Mason University

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0.0 Some Basic Mathematical Concepts 641<br />

Now for finite intersections <strong>of</strong> the open intervals and finite unions <strong>of</strong> the<br />

closed intervals, that is, for finite k, we have<br />

and<br />

k<br />

<br />

a − 1<br />

<br />

1<br />

, b +<br />

i i<br />

i=1<br />

k<br />

<br />

a + 1<br />

<br />

1<br />

, b −<br />

i i<br />

i=1<br />

is open<br />

is closed.<br />

Infinite intersections and unions behave differently with regard to collections<br />

<strong>of</strong> open and closed sets. With the open and closed intervals <strong>of</strong> the special<br />

forms, for infinite intersections and unions, we have the important facts:<br />

and<br />

∞<br />

<br />

a − 1<br />

<br />

1<br />

, b + = [a, b] (0.0.43)<br />

i i<br />

i=1<br />

∞<br />

<br />

a + 1<br />

<br />

1<br />

, b −<br />

i i<br />

i=1<br />

=]a, b[. (0.0.44)<br />

These equations follow from the definitions <strong>of</strong> intersections and unions. To see<br />

equation (0.0.44), for example, we note that a ∈ ∪Ai iff a ∈ Ai for some i;<br />

hence, if a ∈ Ai for any i, then a ∈ ∪Ai.<br />

Likewise, we have<br />

n→∞<br />

i=1<br />

∞<br />

<br />

a + 1<br />

<br />

, b<br />

i<br />

i=1<br />

=<br />

∞<br />

i=1<br />

<br />

a, b + 1<br />

<br />

i<br />

= ]a, b]. (0.0.45)<br />

From this we see that<br />

n<br />

<br />

lim a + 1<br />

<br />

1<br />

, b − =<br />

i i<br />

<br />

lim a +<br />

i→∞<br />

1<br />

<br />

1<br />

, b − . (0.0.46)<br />

i i<br />

Equations (0.0.44) and (0.0.45) for ]a, b[ and ]a, b] above show that open<br />

intervals and half-open intervals are not compact, because no finite collection<br />

<strong>of</strong> sets in the unions cover the intervals.<br />

Intervals in IR d<br />

“Intervals” in IR d are merely product sets <strong>of</strong> intervals in IR; that is, they are<br />

hyperrectangles. Because <strong>of</strong> the possibilities <strong>of</strong> the types <strong>of</strong> endpoints <strong>of</strong> the<br />

intervals in IR, the intervals in IR d cannot always be specified using “[, ]” “], [”,<br />

and so on. In more restrictive cases in which all <strong>of</strong> the intervals in IR are <strong>of</strong><br />

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

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