1. ## sandwich measurable set

Suppose that A is subset of R (real line) with the property for every ε > 0 there are measurable sets B and C s.t. B⊂A⊂C and m(C\B)<ε
Prove A is measurable
By definition A is measurable we need to prove m(E)=m(E∩A)+m(E\A) for all E
the ≤ is trivial enough to show ≥:
Since C is measurable then m(E)= m(E∩C)+m(E\C)
≥ m(E∩A)+m(E\A)-ε (Since A is subset of C)
then move the ε to LHS and since for every ε so, let ε->0 , obtain the result.
is my solution right??? i thought i should use B and m(C\B)<ε somewhere. Could some one help me to check it??
many thanks

2. ## Re: sandwich measurable set

How do you know that $m(E\cap C) + m(E\setminus C) \ge m(E\cap A) + m(E\setminus A) - \varepsilon$? You just stated what you need to prove, but did not prove it.

You need to show that for all $\varepsilon>0$ and for all measurable sets $E\subseteq \mathbb{R}$, $\left|m(E\cap A) + m(E\setminus A) - m(E) \right| < \varepsilon$. So, you need to come up with a way of rewriting $m(E\cap A)+m(E\setminus A)$ using the terms you know something about: $m(E\cap B), m(E\cap C), m(E\setminus B), m(E\setminus C)$.

3. ## Re: sandwich measurable set

I tried the way u provided, but i cannot get anything relatives to m(C\B)< ε . Could you tell me the proof in details???

4. ## Re: sandwich measurable set

Since $m(E\cap A) \le \inf\{m^*(E \cap K) \mid A \subseteq K, K\mbox{ is measurable} \}$ and $m(E \setminus A) \le \inf\{m^*(E \setminus K) \mid K \subseteq A, K\mbox{ is measurable} \}$, you can conclude, $m(E \cap A) + m(E \setminus A) \le m(E \cap C) + m(E \setminus B)$. But, $E \cap C = E \cap (B \cup (C \setminus B))$, so $m(E \cap C) = m(E \cap (B \cup (C\setminus B))) = m((E \cap B) \cup (E \cap (C\setminus B)))$. Since $B,C$ are measurable, use the properties of a $\sigma$-algebra to show that $E\cap B, E\cap (C\setminus B)$ are both measurable. Hence $m((E \cap B) \cup (E \cap (C\setminus B))) = m(E \cap B) + m(E \cap (C\setminus B))$.

To recap, we have
\begin{align*}m(E \cap A) + m(E \setminus A) & \le m(E \cap C) + m(E \setminus B) \\ & = m(E \cap B) + m(E \cap (C \setminus B)) + m(E \setminus B) \\ & = (m(E\cap B) + m(E\setminus B)) + m(E \cap (C\setminus B)) \\ & \le (m(E\cap B) + m(E\setminus B)) + m(C\setminus B) \\ & = m(E) + m(C\setminus B) \\ & < m(E) + \varepsilon\end{align*}

This is because $E \cap (C\setminus B) \subseteq C\setminus B$, so $m(E\cap (C\setminus B)) \le m(C \setminus B)$ and since $B$ is measurable, $m(E) = m(E\cap B) + m(E\setminus B)$.

So, finally, we have $m(E\cap A) + m(E \setminus A) - m(E) < \varepsilon$.