7.2. Which permutations lie in the centralizer of (1 2 3) in S5?

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
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Could you explain how to show 7.2 in detail? I also included lists of definitions and theorems in the book as a reference.

Question : 7.2 Which permutations lie in the centralizer of (1 2 3) in S5?

**7.2.** Which permutations lie in the centralizer of \((1\ 2\ 3)\) in \(S_5\)?

**7.8.** If \(a \in G\) and \(a\) has odd order, show that \(C(a) = C(a^4)\).
Transcribed Image Text:**7.2.** Which permutations lie in the centralizer of \((1\ 2\ 3)\) in \(S_5\)? **7.8.** If \(a \in G\) and \(a\) has odd order, show that \(C(a) = C(a^4)\).
**Centralizers and Normalizers in Group Theory**

**Definition 7.1:** Let \( G \) be a group, \( a \in G \), and \( H \) a subgroup of \( G \). The centralizer of \( a \) is the set of all elements of \( G \) that commute with \( a \). We denote this by \( C(a) = \{ g \in G : ag = ga \} \). Similarly, the centralizer of \( H \) is \( C(H) = \{ g \in G : gh = hg \text{ for all } h \in H \} \).

**Example 7.1:** If \( a \in Z(G) \), then \( C(a) = G \). If \( H \leq Z(G) \), then \( C(H) = G \). Notably, \( C(e) = G \), illustrating that centralizers aren't necessarily abelian.

**Example 7.2:** Let \( G = D_8 \). Here, \( C(R_{270}) = \langle R_{90} \rangle \), \( C(R_{180}) = G \), and \( C(F_1) = \{ R_0, R_{180}, F_1, F_2 \} \).

**Theorem 7.1:** Let \( G \) be a group, \( a \in G \), and \( H \) a subgroup.

1. \( C(H) = \bigcap_{h \in H} C(h) \).
2. \( C(a) \) and \( C(H) \) are both subgroups of \( G \).
3. If \( H \) is a normal subgroup of \( G \), then so is \( C(H) \).
4. \( Z(G) \) is a subgroup of both \( C(a) \) and \( C(H) \).
5. \( C(a) = C(\langle a \rangle) \).

**Definition 7.2:** Let \( G \) be a group and \( H \) a subgroup. The normalizer of \( H \) is the set \( N(H) = \{ a \in G : a^{-1}Ha = H \} \). If \( K \) is another subgroup of \( G \), then \( N_K(H) = N
Transcribed Image Text:**Centralizers and Normalizers in Group Theory** **Definition 7.1:** Let \( G \) be a group, \( a \in G \), and \( H \) a subgroup of \( G \). The centralizer of \( a \) is the set of all elements of \( G \) that commute with \( a \). We denote this by \( C(a) = \{ g \in G : ag = ga \} \). Similarly, the centralizer of \( H \) is \( C(H) = \{ g \in G : gh = hg \text{ for all } h \in H \} \). **Example 7.1:** If \( a \in Z(G) \), then \( C(a) = G \). If \( H \leq Z(G) \), then \( C(H) = G \). Notably, \( C(e) = G \), illustrating that centralizers aren't necessarily abelian. **Example 7.2:** Let \( G = D_8 \). Here, \( C(R_{270}) = \langle R_{90} \rangle \), \( C(R_{180}) = G \), and \( C(F_1) = \{ R_0, R_{180}, F_1, F_2 \} \). **Theorem 7.1:** Let \( G \) be a group, \( a \in G \), and \( H \) a subgroup. 1. \( C(H) = \bigcap_{h \in H} C(h) \). 2. \( C(a) \) and \( C(H) \) are both subgroups of \( G \). 3. If \( H \) is a normal subgroup of \( G \), then so is \( C(H) \). 4. \( Z(G) \) is a subgroup of both \( C(a) \) and \( C(H) \). 5. \( C(a) = C(\langle a \rangle) \). **Definition 7.2:** Let \( G \) be a group and \( H \) a subgroup. The normalizer of \( H \) is the set \( N(H) = \{ a \in G : a^{-1}Ha = H \} \). If \( K \) is another subgroup of \( G \), then \( N_K(H) = N
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