For each of the following groups \(G\text{,}\) determine whether \(H\) is a normal subgroup of \(G\text{.}\) If \(H\) is a normal subgroup, write out a Cayley table for the factor group \(G/H\text{.}\)
Find all the subgroups of \(D_4\text{.}\) Which subgroups are normal? What are all the factor groups of \(D_4\) up to isomorphism?
3.
Find all the subgroups of the quaternion group, \(Q_8\text{.}\) Which subgroups are normal? What are all the factor groups of \(Q_8\) up to isomorphism?
4.
Let \(T\) be the group of nonsingular upper triangular \(2 \times 2\) matrices with entries in \({\mathbb R}\text{;}\) that is, matrices of the form
\begin{equation*}
\begin{pmatrix}
a & b \\
0 & c
\end{pmatrix}\text{,}
\end{equation*}
where \(a\text{,}\)\(b\text{,}\)\(c \in {\mathbb R}\) and \(ac \neq 0\text{.}\) Let \(U\) consist of matrices of the form
Show that the intersection of two normal subgroups is a normal subgroup.
6.
If \(G\) is abelian, prove that \(G/H\) must also be abelian.
7.
Prove or disprove: If \(H\) is a normal subgroup of \(G\) such that \(H\) and \(G/H\) are abelian, then \(G\) is abelian.
8.
If \(G\) is cyclic, prove that \(G/H\) must also be cyclic.
Hint.
If \(a \in G\) is a generator for \(G\text{,}\) then \(aH\) is a generator for \(G/H\text{.}\)
9.
Prove or disprove: If \(H\) and \(G/H\) are cyclic, then \(G\) is cyclic.
10.
Let \(H\) be a subgroup of index \(2\) of a group \(G\text{.}\) Prove that \(H\) must be a normal subgroup of \(G\text{.}\) Conclude that \(S_n\) is not simple for \(n \geq 3\text{.}\)
11.
If a group \(G\) has exactly one subgroup \(H\) of order \(k\text{,}\) prove that \(H\) is normal in \(G\text{.}\)
Hint.
For any \(g \in G\text{,}\) show that the map \(i_g : G \to G\) defined by \(i_g : x \mapsto gxg^{-1}\) is an isomorphism of \(G\) with itself. Then consider \(i_g(H)\text{.}\)
12.
Define the centralizer of an element \(g\) in a group \(G\) to be the set
\begin{equation*}
C(g) = \{ x \in G : xg = gx \}\text{.}
\end{equation*}
Show that \(C(g)\) is a subgroup of \(G\text{.}\) If \(g\) generates a normal subgroup of \(G\text{,}\) prove that \(C(g)\) is normal in \(G\text{.}\)
Hint.
Suppose that \(\langle g \rangle\) is normal in \(G\) and let \(y\) be an arbitrary element of \(G\text{.}\) If \(x \in C(g)\text{,}\) we must show that \(y x y^{-1}\) is also in \(C(g)\text{.}\) Show that \((y x y^{-1}) g = g (y x y^{-1})\text{.}\)
13.
Recall that the center of a group \(G\) is the set
\begin{equation*}
Z(G) = \{ x \in G : xg = gx \text{ for all } g \in G \}\text{.}
\end{equation*}
Calculate the center of \(S_3\text{.}\)
Calculate the center of \(GL_2 ( {\mathbb R} )\text{.}\)
Show that the center of any group \(G\) is a normal subgroup of \(G\text{.}\)
If \(G / Z(G)\) is cyclic, show that \(G\) is abelian.
14.
Let \(G\) be a group and let \(G' = \langle aba^{- 1} b^{-1} \rangle\text{;}\) that is, \(G'\) is the subgroup of all finite products of elements in \(G\) of the form \(aba^{-1}b^{-1}\text{.}\) The subgroup \(G'\) is called the commutator subgroup of \(G\text{.}\)
Show that \(G'\) is a normal subgroup of \(G\text{.}\)
Let \(N\) be a normal subgroup of \(G\text{.}\) Prove that \(G/N\) is abelian if and only if \(N\) contains the commutator subgroup of \(G\text{.}\)
Hint.
(a) Let \(g \in G\) and \(h \in G'\text{.}\) If \(h = aba^{-1}b^{-1}\text{,}\) then
We also need to show that if \(h = h_1 \cdots h_n\) with \(h_i = a_i b_i a_i^{-1} b_i^{-1}\text{,}\) then \(ghg^{-1}\) is a product of elements of the same type. However, \(ghg^{-1} = g h_1 \cdots h_n g^{-1} = (gh_1g^{-1})(gh_2g^{-1}) \cdots (gh_ng^{-1})\text{.}\)
