# Math4121 Lecture 30

## Lebesgue Measure

$\mathfrak{M}=\{S\subseteq\mathbb{R}:S\text{ is Lebesgue measurable}\}$ is a $\sigma$-algebra on $\mathbb{R}$ (closed under complementation and countable unions).

### Consequence of Lebesgue Measure

Every open set and closed set is Lebesgue measurable.

#### Inner and Outer Regularity of Lebesgue Measure

Outer regularity:
$$
m_e(S)=\inf_{U\text{ open},S\subseteq U}m(U)
$$

Inner regularity:
$$
m_i(S)=\sup_{K\text{ closed},K\subseteq S}m(K)
$$

<details>
<summary>Proof</summary>

Inner regularity:

Since $m_i(S)=m(I)-m_e(I\setminus S)$, $S\subseteq I$ for some closed interval $I$. Let $\epsilon>0$ and $U$ be an open set such that $I\setminus S\subseteq U$ and $m(U)<m(I\setminus S)+\epsilon$.

Take $K=I\setminus U$. Then $K\subseteq S$ and $K$ is closed and

$$
m(K)=m(I)-m(U)>m(I)-m(I\setminus S)-\epsilon
$$

So $m_i(S)<m(K)+\epsilon$. Since $\epsilon$ is arbitrary, $m_i(S)\leq m_e(S)$.

</details>

We can approximate $m(S)$ from outside by open sets. If we are just concerned with "approximating" $m(S)$, we can use finite union of intervals.

#### Symmetric difference

The symmetric difference of two sets $S$ and $T$ is defined as

$$
S\Delta T=(S\setminus T)\cup(T\setminus S)
$$

_The XOR operation on two sets._

#### Theorem 

If $S\subset I$ is measurable, then for every $\epsilon>0$, $\exists I_1,I_2,\cdots,I_n\subset I$ open intervals such that

$$
m(S\Delta U)<\epsilon
$$

where $U=\bigcup_{j =1}^n I_j$.

<details>
<summary>Proof</summary>

Let $\epsilon>0$ and $m(V)<m(S)+\frac{\epsilon}{2}$. Let $K\subseteq S$ be closed set such that $m(S)-\frac{\epsilon}{2}<m(K)$. $V$ is an open cover of closed and bounded set $K$. By Heine-Borel theorem, $K$ has a finite subcover. Let $I_1,I_2,\cdots,I_n$ be the open intervals in the subcover.

Check:

$$
m(S\Delta U)=m(S\setminus U)+m(U\setminus S)\leq m(S\setminus K)+m(U\setminus S)<\epsilon
$$

</details>

Recall $\{T_j\}_{j=1}^\infty$ are disjoint measurable sets. Then $T=\bigcup_{j=1}^\infty T_j$ is measurable and

$$
m(T)=\sum_{j=1}^\infty m(T_j)
$$

#### Corollary (Better osgood's theorem on Lebesgue measure)

If $S_1\subseteq S_2\subseteq S_3\subseteq\cdots$  are measruable (no need to be closed and bounded) and $S=\bigcup_{j=1}^\infty S_j$, then

$$
m(S)=\lim_{j\to\infty}m(S_j)
$$

Proof:

Let $T_1=S_1$ and $T_i=S_i\setminus S_{i-1}$ for $i\geq 2$. Still have $S=\bigcup_{j=1}^\infty T_j$.

Where $T_i$ are disjoint measurable sets. So $m(S)=\sum_{j=1}^\infty m(T_j)$.

So $\lim_{j\to\infty}m(S_j)=\sum_{j=1}^\infty m(T_j)=m(S)$.

