# Math Help - Borel set

1. ## Borel set

How does one show the following ( $\mathcal{R}$ denotes the one-dimensional Borel-set):
$\mathcal{R} = \sigma(\mathcal{A})$
with $\mathcal{A}=\{]a,b]|a,b \in \mathbb{Q}, a < b\}\cup {\emptyset}$

I know that $\mathcal{R}= \sigma(\{]a,b]|-\infty so I think $\mathcal{A} \subseteq \mathcal{R}$ and if I can show that $\mathcal{A}$ is a $\pi-$ systeme it follows by the $\pi-\lambda$ theorem that $\sigma(\mathcal{A})\subset \mathcal{R}$.

I could do the reverse implication the same way but then I need to prove that $\{]a,b]|-\infty, but I'm struggling with this implication.

Anyone?