Diophantine Approximation of Anergodic Birkhoff Sums over Rotations (2304.00635v1)

Abstract: We study Birkhoff sums over rotations (series of the form $\sum_{r=1}^{N}\phi(r\alpha)$), in which the summed function $\phi$ may be unbounded at the origin. Estimates of these sums have been of significant interest and application in pure mathematics since the late 1890s, but in recent years they have also appeared in numerous areas of applied mathematics, and have enjoyed significant renewed interest. Functions which have been intensively studied include the reciprocals of number theoretical functions such as $\phi(x)=1/{x},1/{{x}},1/\left\Vert x\right\Vert$, and trigonometric functions such as $\phi(x)=\cot\pi x$ or $\left|\csc\pi x\right|$. Classically the Birkhoff sum of each function has been studied in relative isolation using function specific tools, and the results have frequently been restricted to Bachmann-Landau estimates. We introduce here a more general unified theory which is applicable to all of the above functions. The theory uses only elementary tools (no tools of complex analysis), is capable of giving effective results (explicit bounds), and generally matches or improves on previously available results.