The Intersection of Two Subgroups is also a Group Proof | Summary and Q&A

TL;DR

The intersection of subgroups in a group is itself a subgroup.

Key Insights

• 🤬 The symbol for "subgroup" is denoted by ⊆.
• 🇭🇰 The intersection of subgroups H and K is denoted as H ∩ K.
• 🇭🇰 Closure under the group operation means that the product of two elements in H ∩ K remains within H ∩ K.
• 🇭🇰 Closure under inverses ensures that the inverse of any element in H ∩ K also belongs to H ∩ K.
• 🇭🇰 By satisfying the conditions of nonempty intersection, closure under the group operation, and closure under inverses, H ∩ K is confirmed to be a subgroup of G.
• ❓ Understanding the proof requires grasping the concept of subgroups, the closure property, and the definition of intersection.
• 👥 The proof establishes a fundamental result in group theory regarding subgroups and their intersections.

Transcript

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Questions & Answers

Q: What does the symbol for "subgroup" represent?

The symbol for "subgroup" indicates that a certain subset of a group follows the same group structure and contains the identity element and inverses.

Q: How is it shown that the intersection of H and K is not empty?

Since H is a subgroup of G, it contains the identity element (e), and likewise, K being a subgroup of G also contains e. Therefore, the intersection must contain e as well, making it nonempty.

Q: How is closure under the group operation proven for H ∩ K?

If X and Y are elements in both H and K, then by the closure property of H and K, the product XY is also an element of H and K. Hence, XY belongs to the intersection, demonstrating closure under the group operation.

Q: How is closure under inverses shown for H ∩ K?

Assume X is an element in H ∩ K. By definition, X is in H and K. As H and K are closed under inverses, the inverse of X exists in both H and K. Thus, the inverse of X resides in H ∩ K, proving closure under inverses.

Summary & Key Takeaways

• The content explains a proof showing that if H and K are subgroups of a group G, then their intersection (H ∩ K) is also a subgroup of G.

• The proof consists of three steps: showing that the intersection is not empty, proving closure under the group operation, and demonstrating closure under inverses.

• By satisfying these three conditions, it is proven that H ∩ K is indeed a subgroup of G.