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Unformatted text preview: Fourier Series For the motivation of the problem see the previous lecture. Well actually extend the scope some more and address the fol lowing Problem: Given a function f : [ L,L ] R , can we find coefficients a ,a 1 ,a 2 ,... and b 1 ,b 2 ,.. such that f ( x ) = a 2 + X n =1 a n cos nx L + X n =1 b n sin nx L ? If such coefficients exist, the right hand side above is termed the Fourier Expan sion of f . On the way to the (very) positive answer, we need some information: Z L L cos nx L cos mx L dx = if m 6 = n L if m = n Z L L sin nx L sin mx L dx = if m 6 = n L if m = n Z L L cos nx L sin mx L dx = 0 . If f : [ L,L ] R has a representation as a superposition of sines and cosines as in ? , then 2 f ( x ) = a 2 + X n =1 a n cos nx L + X n =1 b n sin nx L 3 f ( x ) cos mx L = a 2 cos mx L + X n =1 a n cos nx L cos mx L + X n =1 b n sin nx L cos mx L 4 Z L L f ( x ) cos mx L dx = a 2 Z L L cos mx L dx + X n =1 a n Z L L cos nx L cos mx L dx + X n =1 b n Z L L sin nx L cos mx L dx 5 Z L L f ( x ) cos mx L dx = a 2 Z L L cos mx L dx  {z } =0 if m 6 =0 , = La if m =0 + X n =1 a n Z L L cos nx L cos mx L dx  {z } =0 if n 6 = m , = L if n = m  {z } = La m + X n =1 b n Z L L sin nx L cos mx L dx  {z } =0 Hence a m = 1 L Z L L f ( x ) cos mx L dx . m = 0 , 1 ,......
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This note was uploaded on 09/07/2010 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas at Austin.
 Spring '08
 Turner

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