Dirichlet Function is Periodic

Theorem

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

$\forall x \in \R: \map D x = \begin{cases} c & : x \in \Q \\ d & : x \notin \Q \end{cases}$

Then $D$ is periodic.

Namely, every non-zero rational number is a periodic element of $D$.

Proof

Let $x \in \R$.

Let $L \in \Q$.

If $x \in \Q$, then:

\(\ds \map D {x + L}\)

\(=\)

\(\ds c\)

Rational Addition is Closed

\(\ds \)

\(=\)

\(\ds \map D x\)

If $x \notin \Q$, then:

\(\ds \map D {x + L}\)

\(=\)

\(\ds d\)

Rational Number plus Irrational Number is Irrational

\(\ds \)

\(=\)

\(\ds \map D x\)

Combining the above two shows that:

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

Hence the result.

$\blacksquare$