On the symmetry of finite sums of exponentials II

Abstract

In this note we extend our study of the rich geometry of the graph of a curve defined as the weighted sum of two exponentials. Let $γ_{a,b}^{s}: [0,1] \rightarrow \mathbb{C}$ be defined as $$γ_{a,b}^s(t) = (1-s) \exp(2 πi a t) + (1+s) \exp(2πi b t) $$ in which $1\leq a < b$ are two positive integers and $s \in [-1,1]$. In the first part we determined the symmetry groups of the graphs of $γ_{a,b}:=γ_{a,b}^0$. The main aim of this note is to study the continuous transition of the graph of the curve when $s$ changes from $-1$ to $1$. As a main result we determine the winding numbers $\mathrm{wind}(γ_{a,b}^s,0)$ for $s \in [-1,1] \setminus \{ 0 \}$ as well as the set of cusp points of each such curve. This sheds further light on our initial symmetry result and provides more non-trivial albeit easy-to-state examples of advanced concepts of geometry and topology.