Fourier mean-square convergence and Parseval
For a complex 2π-periodic Riemann-integrable f, cₙ=(2π)⁻¹∫₋π^πf(x)e⁻ⁱⁿˣdx and s_N=Σ|n|≤N cₙeⁱⁿˣ satisfy ∫|f−s_N|²→0.
Key knowledge
For a complex 2π-periodic Riemann-integrable f, cₙ=(2π)⁻¹∫₋π^πf(x)e⁻ⁱⁿˣdx and s_N=Σ|n|≤N cₙeⁱⁿˣ satisfy ∫|f−s_N|²→0. Parseval gives (2π)⁻¹∫₋π^π|f|²=Σₙ∈Z|cₙ|². This is mean-square convergence, which does not assert pointwise convergence everywhere.
Conditions and scope
- Use the stated complex-coefficient normalization and one full period.
- Riemann integrability on the compact interval includes boundedness; arbitrary generalized functions need another theorem.
Sources · 1
References
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Evidence and review
Section11.8.1 coefficient convention and Theorem11.8.13, section11.8.6, mean-square and energy identities; proof's approximation/Cauchy–Schwarz steps read.
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Entry details
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