Hello,

I have come across the function

$f(t) = \sum_{j=1}^n c_j e^{2 \pi i a_j t}$

with $c_j \in \mathbb{C}$, $c_j\neq 0$ and $a_j\in\mathbb{R}$, $a_j \neq 0$ for $j=1,...,n$, and the $a_j$ distinct. I want to show that $f(t)$ is periodic with least period equal to $1/\gcd a_j$ if the $a_j$ have a common divisor, and not periodic otherwise.

Is anyone aware of a good reference for this question?