Lebesgue measure

Definition

The set function $$\lambda^n:(\mathbb{R}^n,\mathcal{B}(\mathbb{R}^n))\rightarrow\mathbb{R}_{\ge}$$ that assigns every half-open rectangle $$[[a,b))=[a_1,b_1)\times\cdots\times[a_n,b_n)\in\mathcal{J}$$ as follows:

$$\lambda^n\Big([[a,b))\Big)=\Pi^n_{i=1}(b_i-a_i)$$