 Van der Corput lemma (harmonic analysis)

The Van der Corput lemma is a result in harmonic analysis that provides a way to estimate oscillatory integrals, especially integrals of the form: \[ \int e^{i \phi(t)} f(t) \, dt \] where \( \phi(t) \) is a smooth function, and \( f(t) \) is usually a function that is well-behaved (often in \( L^1 \) space).