If the Quotient by the Center is Cyclic, then the Group is Abelian
Problem 18
Let $Z(G)$ be the center of a group $G$.
Show that if $G/Z(G)$ is a cyclic group, then $G$ is abelian.
Add to solve later Add to solve later A Group with a Prime Power Order Elements Has Order a Power of the Prime.
Problem 17
Let $p$ be a prime number. Suppose that the order of each element of a finite group $G$ is a power of $p$. Then prove that $G$ is a $p$-group. Namely, the order of $G$ is a power of $p$.
Add to solve later Any Subgroup of Index 2 in a Finite Group is Normal
Linear Dependent/Independent Vectors of Polynomials
Problem 15
Let $p_1(x), p_2(x), p_3(x), p_4(x)$ be (real) polynomials of degree at most $3$. Which (if any) of the following two conditions is sufficient for the conclusion that these polynomials are linearly dependent?
(a) At $1$ each of the polynomials has the value $0$. Namely $p_i(1)=0$ for $i=1,2,3,4$.
(b) At $0$ each of the polynomials has the value $1$. Namely $p_i(0)=1$ for $i=1,2,3,4$.
(University of California, Berkeley)
Add to solve later Possibilities of the Number of Solutions of a Homogeneous System of Linear Equations
Problem 14
Here is a very short true or false problem.
Select either True or False. Then click “Finish quiz” button.
You will be able to see an explanation of the solution by clicking “View questions” button.
Add to solve later Common Eigenvector of Two Matrices and Determinant of Commutator
Problem 13
Let $A$ and $B$ be $n\times n$ matrices.
Suppose that these matrices have a common eigenvector $\mathbf{x}$.
Show that $\det(AB-BA)=0$.
Read solution
Add to solve later Transpose of a Matrix and Eigenvalues and Related Questions
Problem 12
Let $A$ be an $n \times n$ real matrix. Prove the followings.
(a) The matrix $AA^{\trans}$ is a symmetric matrix.
(b) The set of eigenvalues of $A$ and the set of eigenvalues of $A^{\trans}$ are equal.
(c) The matrix $AA^{\trans}$ is non-negative definite.
(An $n\times n$ matrix $B$ is called non-negative definite if for any $n$ dimensional vector $\mathbf{x}$, we have $\mathbf{x}^{\trans}B \mathbf{x} \geq 0$.)
(d) All the eigenvalues of $AA^{\trans}$ is non-negative.
Add to solve later Nilpotent Matrix and Eigenvalues of the Matrix
Problem 11
An $n\times n$ matrix $A$ is called nilpotent if $A^k=O$, where $O$ is the $n\times n$ zero matrix.
Prove the followings.
(a) The matrix $A$ is nilpotent if and only if all the eigenvalues of $A$ is zero.
(b) The matrix $A$ is nilpotent if and only if $A^n=O$.
Read solution
Add to solve later The Center of a p-Group is Not Trivial
Problem 10
Let $G$ be a group of order $|G|=p^n$ for some $n \in \N$.
(Such a group is called a $p$-group.)
Show that the center $Z(G)$ of the group $G$ is not trivial.
Add to solve later Determinant/Trace and Eigenvalues of a Matrix
Problem 9
Let $A$ be an $n\times n$ matrix and let $\lambda_1, \dots, \lambda_n$ be its eigenvalues.
Show that
(1) $$\det(A)=\prod_{i=1}^n \lambda_i$$
(2) $$\tr(A)=\sum_{i=1}^n \lambda_i$$
Here $\det(A)$ is the determinant of the matrix $A$ and $\tr(A)$ is the trace of the matrix $A$.
Namely, prove that (1) the determinant of $A$ is the product of its eigenvalues, and (2) the trace of $A$ is the sum of the eigenvalues.
Read solution
Add to solve later How to Find a Formula of the Power of a Matrix
Problem 8
Let $A= \begin{bmatrix}
1 & 2\\
2& 1
\end{bmatrix}$.
Compute $A^n$ for any $n \in \N$.
Add to solve later Powers of a Diagonal Matrix
Problem 7
Let $A=\begin{bmatrix}
a & 0\\
0& b
\end{bmatrix}$.
Show that
(1) $A^n=\begin{bmatrix}
a^n & 0\\
0& b^n
\end{bmatrix}$ for any $n \in \N$.
(2) Let $B=S^{-1}AS$, where $S$ be an invertible $2 \times 2$ matrix.
Show that $B^n=S^{-1}A^n S$ for any $n \in \N$
Add to solve later A Group of Linear Functions
Problem 6
Define the functions $f_{a,b}(x)=ax+b$, where $a, b \in \R$ and $a>0$.
Show that $G:=\{ f_{a,b} \mid a, b \in \R, a>0\}$ is a group . The group operation is function composition.
Add to solve later A Linear Transformation Maps the Zero Vector to the Zero Vector
Problem 5
Let $T : \mathbb{R}^n \to \mathbb{R}^m$ be a linear transformation.
Let $\mathbf{0}_n$ and $\mathbf{0}_m$ be zero vectors of $\mathbb{R}^n$ and $\mathbb{R}^m$, respectively.
Show that $T(\mathbf{0}_n)=\mathbf{0}_m$.
(The Ohio State University Linear Algebra Exam)
Add to solve later The Quotient by the Kernel Induces an Injective Homomorphism
Problem 4
Let $G$ and $G’$ be a group and let $\phi:G \to G’$ be a group homomorphism.
Show that $\phi$ induces an injective homomorphism from $G/\ker{\phi} \to G’$.
Add to solve later A Condition that a Commutator Group is a Normal Subgroup
Problem 3
Let $H$ be a normal subgroup of a group $G$.
Then show that $N:=[H, G]$ is a subgroup of $H$ and $N \triangleleft G$.
Here $[H, G]$ is a subgroup of $G$ generated by commutators $[h,k]:=hkh^{-1}k^{-1}$.
In particular, the commutator subgroup $[G, G]$ is a normal subgroup of $G$
Add to solve later Similar Matrices Have the Same Eigenvalues
Problem 2
Show that if $A$ and $B$ are similar matrices, then they have the same eigenvalues and their algebraic multiplicities are the same.
Add to solve later Invertible Idempotent Matrix is the Identity Matrix
Problem 1
A square matrix $A$ is called idempotent if $A^2=A$.
Show that a square invertible idempotent matrix is the identity matrix.
Add to solve later 
