Editing Dirichlet function (section)

== Periodicity == 

For any real number {{mvar|x}} and any positive rational number {{mvar|T}}, <math>\mathbf{1}_\Q(x + T) = \mathbf{1}_\Q(x)</math>. The Dirichlet function is therefore an example of a real [[periodic function]] which is not [[constant function|constant]] but whose set of periods, the set of rational numbers, is a [[Dense set|dense subset]] of <math>\R</math>.