A Rigidity Phenomenon for the Hardy-Littlewood Maximal Function

The Hardy-Littlewood maximal function $\mathcal{M}$ and the trigonometric function $\sin{x}$ are two central objects in harmonic analysis. We prove that $\mathcal{M}$ characterizes $\sin{x}$ in the following way: let $f \in C^{\alpha}(\mathbb{R}, \mathbb{R})$ be a periodic function and $\alpha > 1/2$. If there exists a real number $0 < \gamma < \infty$ such that the averaging operator $$(A_xf)(r) = \frac{1}{2r}\int_{x-r}^{x+r}{f(z)dz}$$ has a critical point in $r = \gamma$ for every $x \in \mathbb{R}$, then $$f(x) = a+b\sin{(cx + d)} \qquad \mbox{for some}~a,b,c,d \in \mathbb{R}.$$ This statement can be used to derive a characterization of trigonometric functions as those nonconstant functions for which the computation of the maximal function $\mathcal{M}$ is as simple as possible. The proof uses the Lindemann-Weierstrass theorem from transcendental number theory.