Prove that the intersection of two subgroups of G is itself a subgroup of G.

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Author:Erwin Kreyszig
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**Proof: Intersection of Two Subgroups**

**Statement:** Prove that the intersection of two subgroups of \( G \) is itself a subgroup of \( G \).

**Proof Outline:**

To prove this, we need to show that the intersection of two subgroups, say \( H \) and \( K \), of a group \( G \) is also a subgroup of \( G \). Let's denote the intersection by \( H \cap K \).

**1. Non-Empty Intersection:**
   - Since both \( H \) and \( K \) are subgroups, they both contain the identity element \( e \) of \( G \).
   - Therefore, the identity element \( e \) is also in \( H \cap K \).

**2. Closure:**
   - Take any two elements \( a, b \in H \cap K \).
   - Since \( a \) and \( b \) are in both \( H \) and \( K \), and both \( H \) and \( K \) are subgroups, the product \( ab \) is in \( H \) and \( ab \) is in \( K \).
   - Thus, \( ab \) is in \( H \cap K \).

**3. Inverses:**
   - Take any element \( a \in H \cap K \).
   - Since \( a \) is in both \( H \) and \( K \), and both \( H \) and \( K \) are subgroups, the inverse \( a^{-1} \) is in \( H \) and \( a^{-1} \) is in \( K \).
   - Thus, \( a^{-1} \) is in \( H \cap K \).

By satisfying these conditions (non-empty intersection, closure, and inverses), \( H \cap K \) is a subgroup of \( G \). 

This completes the proof.
Transcribed Image Text:**Proof: Intersection of Two Subgroups** **Statement:** Prove that the intersection of two subgroups of \( G \) is itself a subgroup of \( G \). **Proof Outline:** To prove this, we need to show that the intersection of two subgroups, say \( H \) and \( K \), of a group \( G \) is also a subgroup of \( G \). Let's denote the intersection by \( H \cap K \). **1. Non-Empty Intersection:** - Since both \( H \) and \( K \) are subgroups, they both contain the identity element \( e \) of \( G \). - Therefore, the identity element \( e \) is also in \( H \cap K \). **2. Closure:** - Take any two elements \( a, b \in H \cap K \). - Since \( a \) and \( b \) are in both \( H \) and \( K \), and both \( H \) and \( K \) are subgroups, the product \( ab \) is in \( H \) and \( ab \) is in \( K \). - Thus, \( ab \) is in \( H \cap K \). **3. Inverses:** - Take any element \( a \in H \cap K \). - Since \( a \) is in both \( H \) and \( K \), and both \( H \) and \( K \) are subgroups, the inverse \( a^{-1} \) is in \( H \) and \( a^{-1} \) is in \( K \). - Thus, \( a^{-1} \) is in \( H \cap K \). By satisfying these conditions (non-empty intersection, closure, and inverses), \( H \cap K \) is a subgroup of \( G \). This completes the proof.
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