Let 2 be a nonempty set, x EN be a fixed element, and B be a ring on N Define H : B → R+ by µ(A) XA(x) for A E B. Here XA is the characteristic function of A. Show that µ is a measure. Remark: u is called the point measure at x or a Dirac measure. It is often denoted by dp.

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Chapter2: Second-order Linear Odes
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## Question:

Let \( \Omega \) be a nonempty set, \( x \in \Omega \) be a fixed element, and \( \mathcal{B} \) be a ring on \( \Omega \). Define \( \mu : \mathcal{B} \rightarrow \mathbb{R}_+ \) by \( \mu(A) = \chi_A(x) \) for \( A \in \mathcal{B} \). Here \( \chi_A \) is the characteristic function of \( A \).

Show that \( \mu \) is a measure.

**Remark:** \( \mu \) is called the point measure at \( x \) or a Dirac measure. It is often denoted by \( \delta_x \).

---

## Explanation:

If \( \Omega \) is a nonempty set and \( \mathcal{B} \subseteq \mathcal{P}(\Omega) \) is nonempty, then \( \mu : \mathcal{B} \rightarrow \mathbb{R}_\infty \) is called a set function.

**Definition:** Let \( \mathcal{B} \) be a ring on \( \Omega \) and \( \mu : \mathcal{B} \rightarrow \mathbb{R}_\infty \).

- \( \mu \) is called **additive** if \( \mu(A \uplus B) = \mu(A) + \mu(B) \) for all disjoint \( A, B \in \mathcal{B} \).

- \( \mu \) is called **σ-additive** if
  \[
  \mu\left(\biguplus_{n=1}^\infty A_n\right) = \sum_{n=1}^\infty \mu(A_n)
  \]
  for any sequence \( (A_n) \) of pairwise disjoint sets in \( \mathcal{B} \) with \( \biguplus_{n=1}^\infty A_n \in \mathcal{B} \).

- \( \mu \) is called **non-negative** if \( \mu(A) \geq 0 \) for all \( A \in \mathcal{B} \).

- \( \mu \) is called **increasing** if \( \mu(A) \leq
Transcribed Image Text:## Question: Let \( \Omega \) be a nonempty set, \( x \in \Omega \) be a fixed element, and \( \mathcal{B} \) be a ring on \( \Omega \). Define \( \mu : \mathcal{B} \rightarrow \mathbb{R}_+ \) by \( \mu(A) = \chi_A(x) \) for \( A \in \mathcal{B} \). Here \( \chi_A \) is the characteristic function of \( A \). Show that \( \mu \) is a measure. **Remark:** \( \mu \) is called the point measure at \( x \) or a Dirac measure. It is often denoted by \( \delta_x \). --- ## Explanation: If \( \Omega \) is a nonempty set and \( \mathcal{B} \subseteq \mathcal{P}(\Omega) \) is nonempty, then \( \mu : \mathcal{B} \rightarrow \mathbb{R}_\infty \) is called a set function. **Definition:** Let \( \mathcal{B} \) be a ring on \( \Omega \) and \( \mu : \mathcal{B} \rightarrow \mathbb{R}_\infty \). - \( \mu \) is called **additive** if \( \mu(A \uplus B) = \mu(A) + \mu(B) \) for all disjoint \( A, B \in \mathcal{B} \). - \( \mu \) is called **σ-additive** if \[ \mu\left(\biguplus_{n=1}^\infty A_n\right) = \sum_{n=1}^\infty \mu(A_n) \] for any sequence \( (A_n) \) of pairwise disjoint sets in \( \mathcal{B} \) with \( \biguplus_{n=1}^\infty A_n \in \mathcal{B} \). - \( \mu \) is called **non-negative** if \( \mu(A) \geq 0 \) for all \( A \in \mathcal{B} \). - \( \mu \) is called **increasing** if \( \mu(A) \leq
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