Rapid distributional decay of periodic oscillations ($\left|{\int }_0^1\overline{f}(jx)\varphi (x)dx\right|\le j^{-N}\Vert f{\Vert }_1\Vert {\varphi }^{(N)}{\Vert }_{\infty }$)

If $f\in L^1(0,1)$ has mean zero, $j$ is a positive integer and $\varphi \in \mathcal{D}(0,1)$, then for every integer $N\ge 1$,Here the overline denotes the periodic extension, not complex conjugation. Let $h_N=P^{N}f$ use periodic primitives with zero mean. Repeated integration by parts gives the signed identityThere are no boundary terms, and integration over exactly $j$ periods gives $\Vert {\overline{h}}_N(j\cdot ){\Vert }_1=\Vert h_N{\Vert }_1\le \Vert f{\Vert }_1$. This proves the estimate. If zero-mean $g_j$ satisfy $\Vert g_j{\Vert }_1=O(j^m)$ for fixed real $m\ge 0$, choose an integer $N>m$ to obtain ${\overline{g}}_j(j\cdot )\to 0$ in weak convergence of distributions.