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Mathematical Methods for Physicists: A concise introduction - Site Map

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PARSEVAL'S IDENTITY FOR FOURIER INTEGRALS<br />

Figure 4.21.<br />

A damped sine wave.<br />

Its Fourier trans<strong>for</strong>m g…!† is<br />

g…!† ˆp<br />

1<br />

2<br />

Z 1<br />

1<br />

Z 1<br />

ˆ p 1<br />

2<br />

ˆ 1 <br />

p<br />

2 <br />

2<br />

1<br />

f …t†e i!t dt<br />

e t=T e i!t sin ! 0 tdt<br />

1<br />

!‡! 0 i=T 1<br />

! ! 0 i=T<br />

If f …t† is a radiated electric ®eld, the radiated power is proportional to j f …t†j 2<br />

and the total energy radiated is proportional to R 1<br />

0<br />

jf …t† j 2 dt. This is equal to<br />

R 1<br />

0<br />

jg…!†<br />

j 2 d! by Parseval's identity. Then jg…!†j 2 must be the energy radiated<br />

per unit frequency interval.<br />

Parseval's identity can be used to evaluate some de®nite integrals. As an example,<br />

let us revisit Example 4.8, where the given function is<br />

(<br />

f …x† ˆ 1 jxj<br />

< a<br />

0 jxj > a<br />

and its Fourier trans<strong>for</strong>m is<br />

By Parseval's identity, we have<br />

Z 1<br />

1<br />

r<br />

2 sin !a<br />

g…!† ˆ<br />

! :<br />

ff …x† g 2 dx ˆ<br />

187<br />

Z 1<br />

1<br />

fg…!†<br />

g 2 d!:<br />

<br />

:

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