# Doubly and triply periodic functions

A function f is periodic if there exists a constant period ω such that f(x) = f(x + ω) for all x. For example, sine and cosine are periodic with period 2π.

There’s only one way a function on the real line can be periodic. But if you think of functions of a complex variable, it makes sense to look at functions that are periodic in two different directions. Sine and cosine are periodic as you move horizontally across the complex plane, but not if you move in any other direction. But you could imagine a function that’s periodic vertically as well as horizontally.

A doubly periodic function satisfies f(x) = f(x + ω1) and f(x) = f(x + ω2) for all x and for two different fixed complex periods, ω1 and ω2, with different angular components, i.e. the two periods are not real multiples of each other. For example, the two periods could be 1 and i.

How many doubly periodic functions are there? The answer depends on how much regularity you require. If you ask that the functions be differentiable everywhere as functions of a complex variable (i.e. entire), the only doubly periodic functions are constant functions . But if you relax your requirements to allow functions to have singularities, there’s a wide variety of functions that are doubly periodic. These are the elliptic functions. They’re periodic in two independent directions, and meromorphic (i.e. analytic except at isolated poles). 

What about triply periodic functions? If you require them to be meromorphic, then the only triply periodic functions are constant functions. To put it another way, if a meromorphic function is periodic in three directions, it’s periodic in every direction for every period, i.e. constant. If a function has three independent periods, you can construct a sequence with a limit point where the function is constant, and so it’s constant everywhere.