- For 2L periodic functions, an=1L∫L−Lf(x)cosnxdx for n≥1 and bn=1L∫L−Lf(x)sinnxdx for n≥1.
- ˆf(0)=a02 and for n≥1, ˆf(n)=an−ibn2, ˆf(−n)=an+ibn2, which is easily shown by expanding a02+n∑k=1(akcoskx+bksinkx).
where ˆf:=(ˆf(0),ˆf(1),ˆf(−1),ˆf(2),ˆf(−2),…)∈ℓ2 has the same norm as that of f. Thus we say that the linear map f↦ˆf:L2(T)→ℓ2 is isometric. Ok, let's prove that this is equivalent to (for real f∈L2[−L,L]) 12L∫L−L|f|2=(a0)24+∞∑n=1a2n+b2n2.
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