I vaguely remember that the real function defined as:
$$\forall x \in \mathbb R: f(x) = \begin{cases} 1 & : \text{$x$ is rational} \\ 0 & : \text {$x$ is irrational} \end{cases}$$
has a name attached to it, but I can't remember whose. Is it one of the Frenchmen whose names begin with L? I have a vague recollection but can't remember it.