The circle method and pointwise ergodic theorems

The purpose of this article is to discuss the circle method and its quantitative role in understanding pointwise almost everywhere convergence phenomena for polynomial ergodic averaging operators. Specifically, we will use the circle method to illustrate that pointwise almost everywhere convergence and norm convergence in ergodic theory can have fundamentally different natures. More importantly, these differences may necessitate the use of distinct types of tools, which can sometimes be more intriguing than the original problems themselves.