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

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Sec. 2]_Functional Series_305<br />

In the simplest cases, it is sufficient, when determ<strong>in</strong><strong>in</strong>g the region of<br />

convergence of a series (1), to apply to this series certa<strong>in</strong> convergence tests,<br />

hold<strong>in</strong>g x constant.<br />

Diverges .<br />

n<br />

,%f? Diverges<br />

-3-101 X<br />

Fit*. 104<br />

Example 1. Determ<strong>in</strong>e the region of convergence<br />

x+\ (x+\y (X+\Y (*+iy<br />

TT"1<br />

"<br />

2-2*<br />

~ t"~T^ r" t""'~ t<br />

~<br />

of the series<br />

n- 2" + '<br />

Solution. Denot<strong>in</strong>g by u n the general term of the series, we will have<br />

lim<br />

M-*| I = lim I'+H 11 *'* 1<br />

* Jx+l\ '<br />

\U n \ n^*2n + l<br />

(n+\) |jc| 2<br />

Us<strong>in</strong>g d'Alembert's test, we can assert that the series converges (and converges<br />

absolutely), if '

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