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Sunday, October 2, 2011

note to myself


  • For 2L periodic functions, an=1LLLf(x)cosnxdx for n1 and bn=1LLLf(x)sinnxdx for n1.

  • ˆf(0)=a02 and for n1, ˆf(n)=anibn2, ˆf(n)=an+ibn2, which is easily shown by expanding a02+nk=1(akcoskx+bksinkx).
These two translate the general theory to real-valued function. For example, in the theory of fourier series, we know that for each fL2(T), we have f2:=(f,f)=12π2π0|f|2=nZ|ˆf(n)|2=:ˆf2.

where ˆf:=(ˆf(0),ˆf(1),ˆf(1),ˆf(2),ˆf(2),)2 has the same norm as that of f. Thus we say that the linear map fˆf:L2(T)2 is isometric. Ok, let's prove that this is equivalent to (for real fL2[L,L]) 12LLL|f|2=(a0)24+n=1a2n+b2n2.

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