If we know that the outer Lebesgue measure of the interval $(a,b)$ in $\mathbb R ^n$ is $ \displaystyle{ \prod_{j=1}^{n} (b_j - a_j)}$ find the outer Lebesgue measure of the closed interval $ [a,b]$.
Here it is what I did:
$ a= (a_1, a_2, \cdots ,a_n) $ and $ b=(b_1 ,b_2, \cdots ,b_n)$
$\prod_{j=1}^{n} (b_j - a_j) = m^{*}((a,b)) \leq m^{*}([a,b]) \leq m^{*} ( (a-\epsilon , b+ \epsilon)) \quad \forall \epsilon >0$
But it is $\displaystyle{ m^{*} ( (a-\epsilon , b+ \epsilon)) = \prod_{j=1}^{n} (b_j - a_j +2 \epsilon) = \prod_{j=1}^{n} (b_j - a_j) + A(\epsilon) }$ where $ A(\epsilon)$ are the terms of the product with $ \epsilon$. Since the above holds for all $ \epsilon > 0$ the coclusion follows.
Can you help me write the above in a more elegant way?
Thanks in advance!
edit: Actually I am wondering about the part that I wrote about the terms A(ϵ)