## Group of Order 18 is Solvable

## Problem 118

Let $G$ be a finite group of order $18$.

Show that the group $G$ is solvable.

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Let $G$ be a finite group of order $18$.

Show that the group $G$ is solvable.

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Let $G$ be a finite group and $P$ be a nontrivial Sylow subgroup of $G$.

Let $H$ be a subgroup of $G$ containing the normalizer $N_G(P)$ of $P$ in $G$.

Then show that $N_G(H)=H$.

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Let $G$ and $G’$ be groups and let $f:G \to G’$ be a group homomorphism.

If $H’$ is a normal subgroup of the group $G’$, then show that $H=f^{-1}(H’)$ is a normal subgroup of the group $G$.

Express the vector $\mathbf{b}=\begin{bmatrix}

2 \\

13 \\

6

\end{bmatrix}$ as a linear combination of the vectors

\[\mathbf{v}_1=\begin{bmatrix}

1 \\

5 \\

-1

\end{bmatrix},

\mathbf{v}_2=

\begin{bmatrix}

1 \\

2 \\

1

\end{bmatrix},

\mathbf{v}_3=

\begin{bmatrix}

1 \\

4 \\

3

\end{bmatrix}.\]

(*The Ohio State University, Linear Algebra Exam*)

Let

\[A=\begin{bmatrix}

-1 & 2 \\

0 & -1

\end{bmatrix} \text{ and } \mathbf{u}=\begin{bmatrix}

1\\

0

\end{bmatrix}.\]
Compute $A^{2017}\mathbf{u}$.

(*The Ohio State University, Linear Algebra Exam*)

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Let $A$, $B$ be groups. Let $\phi:B \to \Aut(A)$ be a group homomorphism.

The semidirect product $A \rtimes_{\phi} B$ with respect to $\phi$ is a group whose underlying set is $A \times B$ with group operation

\[(a_1, b_1)\cdot (a_2, b_2)=(a_1\phi(b_1)(a_2), b_1b_2),\]
where $a_i \in A, b_i \in B$ for $i=1, 2$.

Let $f: A \to A’$ and $g:B \to B’$ be group isomorphisms. Define $\phi’: B’\to \Aut(A’)$ by sending $b’ \in B’$ to $f\circ \phi(g^{-1}(b’))\circ f^{-1}$.

\[\require{AMScd}

\begin{CD}

B @>{\phi}>> \Aut(A)\\

@A{g^{-1}}AA @VV{\sigma_f}V \\

B’ @>{\phi’}>> \Aut(A’)

\end{CD}\]
Here $\sigma_f:\Aut(A) \to \Aut(A’)$ is defined by $ \alpha \in \Aut(A) \mapsto f\alpha f^{-1}\in \Aut(A’)$.

Then show that

\[A \rtimes_{\phi} B \cong A’ \rtimes_{\phi’} B’.\]

Let $G$ be a simple group and let $X$ be a finite set.

Suppose $G$ acts nontrivially on $X$. That is, there exist $g\in G$ and $x \in X$ such that $g\cdot x \neq x$.

Then show that $G$ is a finite group and the order of $G$ divides $|X|!$.

Let $A$ and $B$ be $n \times n$ real symmetric matrices. Prove the followings.

**(a)** The product $AB$ is symmetric if and only if $AB=BA$.

**(b)** If the product $AB$ is a diagonal matrix, then $AB=BA$.

Let $p \in \Z$ be a prime number.

Then describe the elements of the Galois group of the polynomial $x^p-2$.

Add to solve laterLet $X$ be a subset of a group $G$. Let $C_G(X)$ be the centralizer subgroup of $X$ in $G$.

For any $g \in G$, show that $gC_G(X)g^{-1}=C_G(gXg^{-1})$.

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Let $\F_p$ be the finite field of $p$ elements, where $p$ is a prime number.

Let $G_n=\GL_n(\F_p)$ be the group of $n\times n$ invertible matrices with entries in the field $\F_p$. As usual in linear algebra, we may regard the elements of $G_n$ as linear transformations on $\F_p^n$, the $n$-dimensional vector space over $\F_p$. Therefore, $G_n$ acts on $\F_p^n$.

Let $e_n \in \F_p^n$ be the vector $(1,0, \dots,0)$.

(The so-called first standard basis vector in $\F_p^n$.)

Find the size of the $G_n$-orbit of $e_n$, and show that $\Stab_{G_n}(e_n)$ has order $|G_{n-1}|\cdot p^{n-1}$.

Conclude by induction that

\[|G_n|=p^{n^2}\prod_{i=1}^{n} \left(1-\frac{1}{p^i} \right).\]

For what value(s) of $a$ does the system have nontrivial solutions?

\begin{align*}

&x_1+2x_2+x_3=0\\

&-x_1-x_2+x_3=0\\

& 3x_1+4x_2+ax_3=0.

\end{align*}

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Let $G$ be a finite group of odd order. Assume that $x \in G$ is not the identity element.

Show that $x$ is not conjugate to $x^{-1}$.

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Let $G$ be a finite group of order $n$ and suppose that $p$ is the smallest prime number dividing $n$.

Then prove that any subgroup of index $p$ is a normal subgroup of $G$.

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Test your understanding of basic properties of matrix operations.

There are **10 True or False Quiz Problems**.

These 10 problems are very common and essential.

So make sure to understand these and don’t lose a point if any of these is your exam problems.

(These are actual exam problems at the Ohio State University.)

You can take the quiz as many times as you like.

The solutions will be given after completing all the 10 problems.

Click the **View question** button to see the solutions.

Find the rank of the following real matrix.

\[ \begin{bmatrix}

a & 1 & 2 \\

1 &1 &1 \\

-1 & 1 & 1-a

\end{bmatrix},\]
where $a$ is a real number.

(*Kyoto University, Linear Algebra Exam*)

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Determine whether the following systems of equations (or matrix equations) described below has no solution, one unique solution or infinitely many solutions and justify your answer.

**(a)** \[\left\{

\begin{array}{c}

ax+by=c \\

dx+ey=f,

\end{array}

\right.

\]
where $a,b,c, d$ are scalars satisfying $a/d=b/e=c/f$.

**(b)** $A \mathbf{x}=\mathbf{0}$, where $A$ is a singular matrix.

**(c)** A homogeneous system of $3$ equations in $4$ unknowns.

**(d) **$A\mathbf{x}=\mathbf{b}$, where the row-reduced echelon form of the augmented matrix $[A|\mathbf{b}]$ looks as follows:

\[\begin{bmatrix}

1 & 0 & -1 & 0 \\

0 &1 & 2 & 0 \\

0 & 0 & 0 & 1

\end{bmatrix}.\]
(*The Ohio State University, Linear Algebra Exam*)

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For which choice(s) of the constant $k$ is the following matrix invertible?

\[A=\begin{bmatrix}

1 & 1 & 1 \\

1 &2 &k \\

1 & 4 & k^2

\end{bmatrix}.\]

(*Johns Hopkins University, Linear Algebra Exam*)

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Determine whether a group $G$ of the following order is simple or not.

(a) $|G|=100$.

(b) $|G|=200$.

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Prove that the quadratic fields $\Q(\sqrt{2})$ and $\Q(\sqrt{3})$ are not isomorphic.

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