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Problems in Mathematical Analysis.pdf - pwp.net.ipl.pt

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Sec. 1. Number Series<br />

Cha<strong>pt</strong>er VIII<br />

SERIES<br />

1. Fundamental conce<strong>pt</strong>s. A number series<br />

is called convergent if its partial sum<br />

00<br />

a,+at +...+att +...= att (1)<br />

2l<br />

has a f<strong>in</strong>ite limit as n > oo. The quantity S= lim S n is then called the sum<br />

n -+ oo<br />

of the series, while the number<br />

is called the rema<strong>in</strong>der of the series. If the limit lim S n does not exist (or is<br />

n -* QO<br />

<strong>in</strong>f<strong>in</strong>ite), the series is then called divergent.<br />

If a series converges, then lim an Q (necessary condition for convergence).<br />

n-*oo<br />

The converse is not true.<br />

For convergence of the series (1) it is necessary and sufficient that for<br />

any positive number e it be possible to choose an N such that for n > N<br />

and for any positive p the follow<strong>in</strong>g <strong>in</strong>equality is fulfilled:<br />

(Cauchifs test).<br />

The convergence or divergence of<br />

subtract a f<strong>in</strong>ite number of its terms.<br />

a series is not violated if we add or<br />

2. Tests of convergence and divergence of positive series.<br />

a) Comparison test I. If

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