Riemann–Lebesgue lemma

The Riemann–Lebesgue lemma is a fundamental result in Fourier analysis, dealing with the behavior of the Fourier coefficients of integrable functions. It asserts that if \( f \) is an integrable function on the real line (or on a finite interval), then the Fourier coefficients of \( f \) tend to zero as the frequency goes to infinity.