## Groups and Group Actions: Lecture 7

In which we think about the link between equivalence relations and partitions, and meet cosets.

• Definition of an equivalence class.
• Definition of a partition of a set.
• Theorem 27: Let $\sim$ be an equivalence relation on a set $S$.  The equivalence classes of $\sim$ partition $S$.  To prove this, we checked that each equivalence class is non-empty, that between them they cover the set (their union is the set), and they are pairwise disjoint, using each of reflexivity, symmetry and transitivity along the way.
• Theorem 28: Let $P$ be a partition of a set $S$.  For $a \in S$, write $P_a$ for the unique part in $P$ with $a \in P_a$.  Define a relation $\sim$ on $S$ by $a \sim b$ if and only if $b \in P_a$.  Then $\sim$ is an equivalence relation.  We checked reflexivity, symmetry and transitivity.
• Corollary 29: There is a bijection between equivalence relations on a set $S$ and partitions of the same set $S$.  This was a consequence of Theorems 27 and 28.
• Definition of $\mathbb{Z}_n$, and of binary operations $+$ and $\times$ on $\mathbb{Z}_n$.
• Lemma 30: The operations $+$ and $\times$ on $\mathbb{Z}_n$ are well defined.  Once we’d worked out what we needed to worry about, it was straightforward to check that in fact we didn’t need to worry.
• Proposition 31:
1.  Take $\overline{x} \in \mathbb{Z}_n$.  Then $\overline{x}$ has a multiplicative inverse in $\mathbb{Z}_n$ if and only if $\mathrm{hcf}(x,n)=1$.
2. If $p$ is prime, then $\mathbb{Z}_p$ is a field.
3. Let $\mathbb{Z}_n^{\times} = \{\overline{x} \in \mathbb{Z}_n : \overline{x}$ has a multiplicative inverse $\}$ be the set of units in $\mathbb{Z}_n$. Then $\mathbb{Z}_n^{\times}$ is a group under multiplication.  The proof is an exercise on Sheet 4.
• Proposition 32 (or 30.5): $(\mathbb{Z}_n,+)$ is an Abelian group.  Moreover, it is cyclic and isomorphic to $C_n$.  In addition, $\times$ is associative and commutative on $\mathbb{Z}_n$, and $\times$ is distributive over $+$.  The proof of this is an exercise.
• Definition of a left coset of a subgroup $H$ in a group $G$.  Definition of the notation $G/H$ for the set of cosets.  Definition of the index of a subgroup in a group.  Definition of right cosets.

### Understanding today’s lecture

Can you prove Propositions 31 and 32?

What are the left cosets of $A_n$ in $S_n$?  What is the index of $A_n$ in $S_n$?

Pick a group and a subgroup.  What are the left cosets?  For example, you could pick a small dihedral group like $D_8$ (the symmetries of a square) and explore left cosets of subgroups of $D_8$.

In Lemma 30, we showed that two functions we’d just written down were well defined.  What does well defined mean?  Here are some useful thoughts from Tim Gowers.

I’ve written before about the result that the non-zero integers modulo a prime form a group under multiplication, which is closely related to $\mathbb{Z}_p$ (for $p$ prime) being a field.

And here’s something a bit lighter, in case you’re feeling old as the end of term approaches.

### Preparation for Lecture 8

Let $G$ and $H$ be isomorphic groups, and let $\theta: G \to H$ be an isomorphism.  Take $g \in G$.  What can you say about the orders of $g$ in $G$ and $\theta(g)$ in $H$?  How are they related?

Can you say anything interesting about the index of a subgroup $H$ in a finite group $G$, in terms of the orders of $G$ and $H$?

Take a finite group $G$, and a subgroup $H$.  Can you show that the left cosets of $H$ partition $G$?  What is the corresponding equivalence relation on the set $G$?

Can you show that all the left cosets of $H$ in $G$ have the same size?

### 4 Responses to “Groups and Group Actions: Lecture 7”

1. Groups and Group Actions: Lecture 8 | Theorem of the week Says:

[…] Expositions of interesting mathematical results « Groups and Group Actions: Lecture 7 […]

2. Groups and Group Actions: Lecture 8.5 | Theorem of the week Says:

[…] come across (left) cosets of a subgroup in a group .  They’re the translates where .  Intriguing question: can we […]

3. Groups and Group Actions: Lecture 9 | Theorem of the week Says:

[…] that are self-inverse — ) or 2 ().  By counting the number of each, and remembering that equivalence classes partition the set, we saw that the number of classes of size 1 is even.  Since it’s also at least 1, there […]

4. Groups and Groups Actions: Lecture 12 | Theorem of the week Says:

[…] Proposition 56: The orbits of an action partition the set.  We defined an equivalence relation whose equivalence classes are precisely the orbits, and were then done by Theorem 27. […]