Dirichlet Function has no Period

Theorem

However, they do not admit a period.

That is, there does not exist a smallest value $L \in \R_{> 0}$ such that:

$\forall x \in \R: \map D x = \map D {x + L}$

Proof

Let $D: \R \to \R$ be a Dirichlet function.

In proving that the Dirichlet Function is Periodic, it was shown that every non-zero rational number is a periodic element of $D$.

Therefore, the period of $D$ must be the smallest element of $\Q_{> 0}$.

But from Rational Numbers are not Well-Ordered under Conventional Ordering there cannot exist such an element.

Hence the result.

$\blacksquare$