how to write naive in latex code example

Example: ensemble latex

\newcommand{\reels}{\mathbb{R}}
$ \forall a \in \reels\quad \forall \varepsilon \in \reels^{*}_{+}\quad 
\exists \eta \in \reels^{*}_{+} \quad 
\forall x \in \reels\quad |x-a| < \eta \implies| f(x) - f(a) | < \varepsilon $