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Theory of Statistics - George Mason University

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644 0 Statistical Mathematics<br />

the order in IR d may require some redefinition <strong>of</strong> the order that arises from<br />

applying the IR order directly (see Exercise 0.0.7).<br />

Theorem 0.0.9 is an alternate statement <strong>of</strong> the Bolzano-Weierstrass theorem.<br />

Theorem 0.0.9 (Bolzano-Weierstrass (alternate))<br />

Every bounded sequence in IR has an accumulation point.<br />

We will prove this statement <strong>of</strong> the theorem directly, because in doing so we<br />

are led to the concept <strong>of</strong> a largest accumulation point, which has more general<br />

uses.<br />

Pro<strong>of</strong>.<br />

For the bounded sequence {xn}, define the set <strong>of</strong> real numbers<br />

S = {x | there are infinitely many xn > x}.<br />

Let x ∗ = sup(S). Because the sequence is bounded, x ∗ is finite. By definition<br />

<strong>of</strong> S and sup(S), for any ɛ > 0, only there are only finitely many xn such that<br />

xn ≥ x ∗ + ɛ, but there are infinitely many xn such that xn ≥ x ∗ − ɛ, so there<br />

are infinitely many xn in the interval [x ∗ − ɛ, x ∗ + ɛ].<br />

Now for i = 1, 2, . . ., consider the intervals Ii = [x ∗ −1/i, x ∗ +1/i] each <strong>of</strong><br />

which contains infinitely many xn, and form a monotone increasing sequence<br />

{ni} such that xni ∈ Ii. (Such a sequence is not unique.) Now use the sequence<br />

{ni} to form a subsequence <strong>of</strong> {xn}, {xni}. The sequence {xni} converges to<br />

x ∗ ; which is therefore an accumulation point <strong>of</strong> {xn}.<br />

lim sup and lim inf<br />

Because <strong>of</strong> the way S was defined in the pro<strong>of</strong> <strong>of</strong> Theorem 0.0.9, the accumulation<br />

point x ∗ = sup(S) is the largest accumulation point <strong>of</strong> {xn}. The<br />

largest accumulation point <strong>of</strong> a sequence is an important property <strong>of</strong> that sequence.<br />

We call the largest accumulation point <strong>of</strong> the sequence {xn} the limit<br />

superior <strong>of</strong> the sequence and denote it as lim sup n xn. If the sequence is not<br />

bounded from above, we define lim sup n xn as ∞. We have<br />

We see that<br />

lim sup<br />

n<br />

xn = lim sup xk. (0.0.47)<br />

n k≥n<br />

lim supxn<br />

= sup{x | there are infinitely many xn > x}, (0.0.48)<br />

n<br />

which is a characterization <strong>of</strong> lim sup for any nonincreasing real point sequence<br />

{xn}. (Compare this with equation (0.0.22) on page 622.)<br />

Likewise, for a bounded sequence, we define the smallest accumulation<br />

point <strong>of</strong> the sequence {xn} the limit inferior <strong>of</strong> the sequence and denote it as<br />

lim infn xn. If the sequence is not bounded from below, we define lim infn xn<br />

as −∞. We have<br />

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

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