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Chapter 10: Problem 6

If \(G\) is abelian, prove that \(G / H\) must also be abelian.

Short Answer

Expert verified
Question: Prove that if a group \(G\) is abelian, then the quotient group \(G / H\) is also abelian. Answer: To prove this, we first define the operation on cosets in the quotient group \(G / H\) as \((aH)(bH) = abH\). Then, because \(G\) is abelian, we have \(ab = ba\), so we can show that \((aH)(bH) = abH = baH = (bH)(aH)\), which means the quotient group \(G / H\) is also abelian.

Step by step solution

01

Define the operation on cosets

Let G be an abelian group and H be a normal subgroup of G. The operation on the quotient group G / H is defined by the cosets multiplication: \((aH)(bH) = abH\) for all \(a,b \in G\).
02

Check the commutative property

Now we need to check if the operation defined above satisfies the commutative property, that is, we need to prove that for any two cosets \((aH)(bH) = (bH)(aH)\). Consider any two cosets \(aH\) and \(bH\) in \(G / H\). Then we have: \((aH)(bH)= abH\) (by the definition of coset multiplication in Step 1) Since \(G\) is abelian, we have \(ab = ba\). Thus, we can rewrite the above equation as: \(abH = baH\) Now, we have: \((bH)(aH) = baH\) Comparing the two equations, we get \((aH)(bH) = (bH)(aH)\). Thus, the quotient group \(G / H\) is also abelian.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Abelian Group
An Abelian group is a special type of group in abstract algebra where the group operation is commutative. This means that for any two elements, say \(a\) and \(b\), in an Abelian group \(G\), the equation \(a + b = b + a\) always holds true.
The notation \(a + b\) is obviously a placeholder for the actual operation in the group, which might be something other than addition (like multiplication or some other operation).
  • The core property of an Abelian group is its commutativity.
  • This makes computations within the group predictable and often simplifies many problems in algebra.
Many familiar groups, such as the integers under addition, are Abelian. The study of these groups helps us understand fundamental structures in mathematics.
Cosets
Cosets are an important concept when dealing with groups and their subgroups. If you have a subgroup \(H\) of a group \(G\), and you pick an element \(a\) from \(G\), you can form a set called a coset.
The left coset, for instance, is denoted as \(aH = \{ah : h \in H\}\) which means every element of \(H\) gets multiplied by \(a\).
  • Cosets divide the group \(G\) into non-overlapping parts.
  • They are crucial in forming quotient groups, since a quotient group \(G/H\) is essentially the set of all cosets of \(H\) in \(G\).
Understanding cosets helps us to perform operations at this higher level, such as finding when a collection of these cosets forms a group.
Normal Subgroup
A normal subgroup \(H\) of a group \(G\) is a subgroup where every left coset of \(H\) is the same as its right coset, meaning \(aH = Ha\) for every \(a\) in \(G\). This symmetry ensures our operations within the quotient group work the way we expect.
  • Every coset looks the same from both sides.
  • Normal subgroups allow the formation of quotient groups, which help to bridge the gap between simpler structures and more complex ones.
In essence, being a normal subgroup tells us that it "fits in nicely" into the larger group \(G\), leading to symmetric properties in quotient group calculations.
Commutative Property
The commutative property is a foundational concept in mathematics and specifically in the study of group theory. It states that the order in which two elements are combined does not affect the result. For a group, this means if you take any two elements \(a\) and \(b\) in the group, the equation \(a b = b a\) holds true.
  • This is true for any two elements within an Abelian group, as Abelian groups are inherently commutative.
  • In the context of quotient groups like \(G/H\), demonstrating the commutative property helps establish whether the quotient group itself is Abelian.
The commutative property is more than just a convenient trait; it is a crucial characteristic that simplifies many aspects of group operations and allows for a deeper understanding of the group's structure.

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Most popular questions from this chapter

Show that the intersection of two normal subgroups is a normal subgroup.

If \(G\) is cyclic, prove that \(G / H\) must also be cyclic.

Recall that the center of a group \(G\) is the set $$Z(G)=\\{x \in G: x g=g x \text { for all } g \in G\\}$$ (a) Calculate the center of \(S_{3}\). (b) Calculate the center of \(G L_{2}(\mathbb{R})\). (c) Show that the center of any group \(G\) is a normal subgroup of \(G\). (d) If \(G / Z(G)\) is cyclic, show that \(G\) is abelian.

For each of the following groups \(G,\) determine whether \(H\) is a normal subgroup of \(G\). If \(H\) is a normal subgroup, write out a Cayley table for the factor group \(G / H\). (a) \(G=S_{4}\) and \(H=A_{4}\) (b) \(G=A_{5}\) and \(H=\\{(1),(123),(132)\\}\) (c) \(G=S_{4}\) and \(H=D_{4}\) (d) \(G=Q_{8}\) and \(H=\\{1,-1, I,-I\\}\) (e) \(G=\mathbb{Z}\) and \(H=5 \mathbb{Z}\)

Define the centralizer of an element \(g\) in a group \(G\) to be the set $$C(g)=\\{x \in G: x g=g x\\}$$ Show that \(C(g)\) is a subgroup of \(G\). If \(g\) generates a normal subgroup of \(G,\) prove that \(C(g)\) is normal in \(G\).
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