# What is the name of the function that equals $1$ when argument rational and $0$ when irrational?

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.