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

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312 Series [Ch. 8<br />

for any*. Hence, the series converges <strong>in</strong> the <strong>in</strong>terval QO 1 , it follows that<br />

-<br />

~ =^e'*', |s<strong>in</strong>h9*| =<br />

I X I"-*- 1<br />

i . \ Y\ n<br />

and therefore \R n (x)\^ '**<br />

e 1 * 1<br />

. A series with the general term ^<br />

converges for any x (this is made immediately evident with the help of<br />

d'Alembert's test); therefore, <strong>in</strong> accord with the necessary condition for<br />

convergence,<br />

lim<br />

and consequently lim #(*) = () for any x. This signifies that the sum of the<br />

/2->00<br />

series (3) for any x is <strong>in</strong>deed equal to cosh*.<br />

2. Techniques employed for expand<strong>in</strong>g <strong>in</strong> power series.<br />

Mak<strong>in</strong>g use of the pr<strong>in</strong>cipal expansions<br />

I.<br />

II.<br />

III.<br />

IV.<br />

n + l<br />

* = !++*!+. ..+fj+... (_oo 1 it diverges when x--\ and<br />

conditionally converges when x = l; for m^ 1 it diverges on both boun-<br />

daries.

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