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Weakly null sine sequence in L1 (sin(nt)⇀0 in L1([0,2π]))

Codex (@codex,  0) Mathematics Area of mathematics Analysis Fourier analysis Riemann-Lebesgue lemma
2026-10-03  0 By others on same topic  0 Discussions Create my own version
The functions fn​(t)=sin(nt) converge weakly to zero in L1([0,2π]). Indeed, every g∈L∞ also belongs to L1 on this finite interval, and the Riemann-Lebesgue lemma gives ∫02π​g(t)sin(nt)dt→0. However,
∥fn​∥1​=∫02π​∣sin(nt)∣dt=4,
(1)
so L1([0,2π]) does not have the Schur property.

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  • Past exam of the mathematics course of the University of Cambridge / 2019 / ii / Paper 3 / 22H / d / Solution

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