8.38. Find the characteristic of each of the following rings. 1. Z4 O Z10 2. M2(Z3)

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Could you explain how to show 8.38 in detail? I also attached definitions and theorems in my textbook.

**Definition 8.13.** Let \( R \) be a ring. Then the **characteristic** of \( R \), denoted char \( R \), is the smallest positive integer \( n \) such that \( na = 0 \) for all \( a \in R \). If no such \( n \) exists, then char \( R = 0 \).

**Example 8.31.** The characteristic of \( \mathbb{Z}_n \) is \( n \), as clearly \( na = 0 \) for any \( a \in \mathbb{Z}_n \), whereas no smaller value than \( n \) will work if we take \( a = 1 \).

**Example 8.32.** The ring of integers has characteristic zero.

In fact, for rings with identity, we only need to look at the identity.

**Theorem 8.13.** Let \( R \) be a ring with identity. Regarding \( R \) as an additive group, if the order of 1 is \( n < \infty \), then \( R \) has characteristic \( n \). If 1 has infinite order, then \( R \) has characteristic zero.

**Corollary 8.2.** Let \( R \) be a ring with identity. Then every unital subring of \( R \) has the same characteristic as \( R \).

**Theorem 8.14 (Freshman’s Dream).** Let \( R \) be a commutative ring of prime characteristic \( p \). Then for any \( a, b \in R \), we have
\[
(a + b)^p = a^p + b^p.
\]

**Theorem 8.15.** The characteristic of an integral domain is either zero or a prime.
Transcribed Image Text:**Definition 8.13.** Let \( R \) be a ring. Then the **characteristic** of \( R \), denoted char \( R \), is the smallest positive integer \( n \) such that \( na = 0 \) for all \( a \in R \). If no such \( n \) exists, then char \( R = 0 \). **Example 8.31.** The characteristic of \( \mathbb{Z}_n \) is \( n \), as clearly \( na = 0 \) for any \( a \in \mathbb{Z}_n \), whereas no smaller value than \( n \) will work if we take \( a = 1 \). **Example 8.32.** The ring of integers has characteristic zero. In fact, for rings with identity, we only need to look at the identity. **Theorem 8.13.** Let \( R \) be a ring with identity. Regarding \( R \) as an additive group, if the order of 1 is \( n < \infty \), then \( R \) has characteristic \( n \). If 1 has infinite order, then \( R \) has characteristic zero. **Corollary 8.2.** Let \( R \) be a ring with identity. Then every unital subring of \( R \) has the same characteristic as \( R \). **Theorem 8.14 (Freshman’s Dream).** Let \( R \) be a commutative ring of prime characteristic \( p \). Then for any \( a, b \in R \), we have \[ (a + b)^p = a^p + b^p. \] **Theorem 8.15.** The characteristic of an integral domain is either zero or a prime.
**Exercise 8.38:**

Find the characteristic of each of the following rings.

1. \( \mathbb{Z}_4 \oplus \mathbb{Z}_{10} \)
2. \( M_2(\mathbb{Z}_3) \)
Transcribed Image Text:**Exercise 8.38:** Find the characteristic of each of the following rings. 1. \( \mathbb{Z}_4 \oplus \mathbb{Z}_{10} \) 2. \( M_2(\mathbb{Z}_3) \)
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