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We've seen that when we create an identification graph of $G$ using right cosets of $H$, we sometimes get a Cayley graph, and we sometimes don't. Today, we'll be focusing on the question, "When is an identification graph of $G$ using cosets of $H$ another Cayley graph?" To answer this question, we'll need to explore the properties of cosets in depth.
When working with the determinant map $f:GL(2,\mathbb{Z}_p)\to U(p)$, we have already shown that the kernel of this map is the special linear group $K=SL(2,\mathbb{Z}_p)$. We've also shown that the coset $Ka$ equals the set of all matrices whose determinant matches the determinant of $a$. Another way to say this is that $Ka$ equals the set of values $b\in G(2,\mathbb{Z}_p)$ such that $f(b)=f(a)$. A similar argument would show that $aK$ is the exact same set, which means we have $Ka=aK$. The next problem has you show that for any homomorphism $f:G\to H$ with kernel $K$, we always have $Ka=aK$ and that $Ka$ equals the set of $b\in G$ such that $f(b)=f(a)$.
Problem 83 (The Right And Left Cosets Of The Kernel Are Equal)
Suppose that $f:G\to H$ is a homomorphism. Let $K$ be the kernel of $f$ and let $a\in G$. Prove the following:
- The right coset $Ka$ equals the set of values $b\in G$ such that $f(b)=f(a)$, or symbolically we have $$Ka = \{b\in G\mid f(b)=f(a)\} = f^{-1}(f(a)).$$
- The left coset $aK$ equals the same set $\{b\in G\mid f(b)=f(a)\}$, which means the left and right cosets are the same or $Ka=aK$.
In the previous problem, we saw that any time $K$ is the kernel of a homomorphism, we can talk about left or right cosets and obtain the exact same result, namely $Ka=aK$ for every $a\in G$. Any time a subgroup of $G$ satisfies this result, we'd like to have a special name for that subgroup. We call these subgroups normal.
Definition (Normal Subgroup $N\trianglelefteq G$)
We say that a subgroup $N$ of $G$ is a normal subgroup of $G$ if for each $a\in G$ the right coset $Na$ and left coset $aN$ are equal, so we have $Na=aN$ for every $a\in G$. We write $N\trianglelefteq G$ to mean that $N$ is a normal subgroup of $G$.
The next theorem should come as no surprise. We've already shown that given any subgroup $H$ of $G$, that every element of $H$ is in one of the cosets of $H$. We've also shown that the cosets of $H$ are disjoint. These two facts together show that we can partition $G$ into a disjoint union of cosets of $H$. We also know that the size of each coset matches the order of $H$. So if we have $r$ disjoint cosets $Ha_1$, $Ha_2$, $Ha_3$, ..., $Ha_r$, all of which have the same number of elements, then we should be able to use this information to show that the order of $H$ divides the order of the group. This fact is called Lagrange's theorem.
Problem 86 (Lagrange's Theorem Proof)
Prove Lagrange's Theorem.
Theorem (Lagrange's Theorem)
Suppose that $G$ is a finite group and that $H$ is a subgroup of $G$. Then the order of $H$ divides the order of $G$. In particular, we know that $|G|/|H|$ equals the number of distinct right (or left) cosets of $H$ in $G$.
We now have shown that the $|G|/|H|$ is precisely the number of cosets of $H$ there are in $G$. From a Cayley graph perspective, this is how many copies of $H$ are visible in the graph, as they are translated throughout the graph. The number of such cosets is called the index of $H$ in $G$, and written $|G:H|$.
Definition (Index of $H$ in $G$, or $|G:H|$)
If $H$ is a subgroup of $G$, then the index of $H$ in $G$, written $|G:H|$, is the number of distinct right (or left) cosets of $H$. Because of Lagrange's theorem, we know that $|G:H|=|G|/|H|$.
We'll now return to looking at when the left and right cosets of a subgroup agree, namely we'll be looking at normal subgroups. Let's first get ourselves a collection of subgroups that are normal.
Problem 87 (Some Normal Subgroups)
Prove the following facts.
- If $G$ is Abelian, then every subgroup of $G$ is normal.
- The center $Z(G)$ of any group $G$ is always a normal subgroup of $G$.
We are now ready to define a new group using the cosets of a normal subgroup. If $N$ is a normal subgroup of $G$, then we've already shown that the binary operation on the collection of cosets, defined by $(Na)(Nb)=N(ab)$, is well defined. Is this binary operation enough to forma a group? To say yes, we need to show that the operation is associative, that there is there an identity coset, and that every coset has an inverse coset. We're now ready to prove that the collection of cosets of a normal subgroup is actually a group, and we call this the factor group of $G$ by $N$, or the quotient group of $G$ by $N$, and we write $G/N$ to talk about this group of cosets.
Definition (Factor Group or Quotient Group of $G$ by $N$, or $G/N$)
Let $G$ be a group. Let $N$ be a normal subgroup of $G$. Then we define the factor group of $G$ by $N$, or the quotient group of $G$ by $N$, to be the set $G/N = \{Na\mid a\in G\}$, i.e the collection of right cosets of $N$ using the set product $(Na)(Nb)=N(ab)$ as the binary operation.
Problem 88 (The Quotient Group $G/N$ Is A Group)
Let $G$ be a group and let $N$ be a normal subgroup of $G$. Prove that the set $G/N$ is actually a group under the operation of coset products.
For more problems, see AllProblems