The Brjuno and Wilton Functions

Abstract: The Brjuno and Wilton functions bear a striking resemblance, despite their very different origins; while the Brjuno function $B(x)$ is a fundamental tool in one-dimensional holomorphic dynamics, the Wilton function $W(x)$ stems from the study of divisor sums and self-correlation functions in analytic number theory. We show that these perspectives are unified by the semi-Brjuno function $B_0(x)$. Namely, $B(x)$ and $W(x)$ can be expressed in terms of the even and odd parts of $B_0(x)$, respectively, up to a bounded defect. Based on numerical observations, we further analyze the arising functions $\Delta^+(x) = B^+(x) - 2B_0^+(x)$ and $\Delta^-(x) = W^-(x) - 2B_0^-(x)$, the first of which is H\"older continuous whereas the second exhibits discontinuities at rationals, behaving similarly to the classical popcorn function.