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

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Sec. 4\__Fourier Series _321<br />

2690.<br />

2691. / (x) = x s<strong>in</strong> x.<br />

when<br />

cos x when < x ^ ~ ,<br />

cos x when Y<br />

2693. Us<strong>in</strong>g the expansions<br />

oi the functions x and x* <strong>in</strong> the<br />

<strong>in</strong>terval (0, it) <strong>in</strong> cos<strong>in</strong>es of mult<strong>ipl</strong>e arcs (see <strong>Problems</strong> 2681 and<br />

2682), prove the equality<br />

2694**. Prove that if the function /(x) is even and we have<br />

?L + x} = f(~ x\ , then its Fourier series <strong>in</strong> the <strong>in</strong>terval<br />

( Ji, n) represents an expansion <strong>in</strong> cos<strong>in</strong>es of odd mult<strong>ipl</strong>e arcs,<br />

and if the function f(x) is odd arid /fy-f *) =/ (y~*) then<br />

<strong>in</strong> the <strong>in</strong>terval ( Ji, Ji)<br />

it is expanded <strong>in</strong> s<strong>in</strong>es of odd mul-<br />

t<strong>ipl</strong>e arcs.<br />

Expand the follow<strong>in</strong>g functions <strong>in</strong> Fourier series <strong>in</strong> the <strong>in</strong>dicated<br />

<strong>in</strong>tervals:<br />

2695. f(x) = \x\<br />

2696. f(x)=*2x<br />

2697. f(x) = e*<br />

( !

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