Exponential sums equations and the Exponential Closedness conjecture

Abstract

This is an expository paper aiming to introduce Zilber's Exponential Closedness conjecture to a general audience. Exponential Closedness predicts when (systems of) equations involving addition, multiplication, and exponentiation have solutions in the complex numbers. It is a natural statement at the boundary between complex geometry and algebraic geometry. While it is open in full generality, many special cases and variants have been proven in the last two decades. In the first part of the paper we give a proof of a special case of the conjecture, namely, we show that exponential sums in a single complex variable, such as $e^{\sqrt{2}z}+3i e^{5z}+\pi$, always have infinitely many zeroes. This follows from well-known results in the literature and can actually be proven by standard complex analytic tools. We present a proof, motivated by the ideas of Zilber and Gallinaro, which uses simplified versions of some ingredients of more advanced techniques in the area, thus allowing the reader to explore these advanced proofs on a simple example. Our approach uses only elementary techniques (from standard undergraduate algebra and complex analysis), and the proofs of some of the main ingredients appear to be new. Moreover, these ingredients come from various areas of mathematics, such as functional transcendence and complex geometry, and are a useful way of introducing some classical concepts in these areas to the reader. In the second part we explain the Exponential Closedness conjecture and discuss some known special cases, not least the so-called ``raising to powers'' case (due to Gallinaro) which generalises the above-mentioned result and whose proof uses advanced versions of the tools presented in the first part of the paper.