Prove that $\mathbf{v} \mathbf{v}^\trans$ is a Symmetric Matrix for any Vector $\mathbf{v}$
Problem 640
Let $\mathbf{v}$ be an $n \times 1$ column vector.
Prove that $\mathbf{v} \mathbf{v}^\trans$ is a symmetric matrix.
Add to solve later Add to solve later The Length of a Vector is Zero if and only if the Vector is the Zero Vector
Problem 639
Let $\mathbf{v}$ be an $n \times 1$ column vector.
Prove that $\mathbf{v}^\trans \mathbf{v} = 0$ if and only if $\mathbf{v}$ is the zero vector $\mathbf{0}$.
Add to solve later A Relation between the Dot Product and the Trace
Problem 638
Let $\mathbf{v}$ and $\mathbf{w}$ be two $n \times 1$ column vectors.
Prove that $\tr ( \mathbf{v} \mathbf{w}^\trans ) = \mathbf{v}^\trans \mathbf{w}$.
Add to solve later Prove that the Dot Product is Commutative: $\mathbf{v}\cdot \mathbf{w}= \mathbf{w} \cdot \mathbf{v}$
Problem 637
Let $\mathbf{v}$ and $\mathbf{w}$ be two $n \times 1$ column vectors.
(a) Prove that $\mathbf{v}^\trans \mathbf{w} = \mathbf{w}^\trans \mathbf{v}$.
(b) Provide an example to show that $\mathbf{v} \mathbf{w}^\trans$ is not always equal to $\mathbf{w} \mathbf{v}^\trans$.
Add to solve later Matrix Operations with Transpose
Problem 636
Calculate the following expressions, using the following matrices:
\[A = \begin{bmatrix} 2 & 3 \\ -5 & 1 \end{bmatrix}, \qquad B = \begin{bmatrix} 0 & -1 \\ 1 & -1 \end{bmatrix}, \qquad \mathbf{v} = \begin{bmatrix} 2 \\ -4 \end{bmatrix}\]
(a) $A B^\trans + \mathbf{v} \mathbf{v}^\trans$.
(b) $A \mathbf{v} – 2 \mathbf{v}$.
(c) $\mathbf{v}^{\trans} B$.
(d) $\mathbf{v}^\trans \mathbf{v} + \mathbf{v}^\trans B A^\trans \mathbf{v}$.
Add to solve later Prove that $(A + B) \mathbf{v} = A\mathbf{v} + B\mathbf{v}$ Using the Matrix Components
Problem 635
Let $A$ and $B$ be $n \times n$ matrices, and $\mathbf{v}$ an $n \times 1$ column vector.
Use the matrix components to prove that $(A + B) \mathbf{v} = A\mathbf{v} + B\mathbf{v}$.
Add to solve later Does the Trace Commute with Matrix Multiplication? Is $\tr (A B) = \tr (A) \tr (B) $?
Problem 634
Let $A$ and $B$ be $n \times n$ matrices.
Is it always true that $\tr (A B) = \tr (A) \tr (B) $?
If it is true, prove it. If not, give a counterexample.
Add to solve later Is the Trace of the Transposed Matrix the Same as the Trace of the Matrix?
Problem 633
Let $A$ be an $n \times n$ matrix.
Is it true that $\tr ( A^\trans ) = \tr(A)$? If it is true, prove it. If not, give a counterexample.
Add to solve later The Vector $S^{-1}\mathbf{v}$ is the Coordinate Vector of $\mathbf{v}$
Problem 632
Suppose that $B=\{\mathbf{v}_1, \mathbf{v}_2\}$ is a basis for $\R^2$. Let $S:=[\mathbf{v}_1, \mathbf{v}_2]$.
Note that as the column vectors of $S$ are linearly independent, the matrix $S$ is invertible.
Prove that for each vector $\mathbf{v} \in V$, the vector $S^{-1}\mathbf{v}$ is the coordinate vector of $\mathbf{v}$ with respect to the basis $B$.
Add to solve later Express the Eigenvalues of a 2 by 2 Matrix in Terms of the Trace and Determinant
Problem 631
Let $A=\begin{bmatrix}
a & b\\
c& d
\end{bmatrix}$ be an $2\times 2$ matrix.
Express the eigenvalues of $A$ in terms of the trace and the determinant of $A$.
Add to solve later Find Eigenvalues, Eigenvectors, and Diagonalize the 2 by 2 Matrix
Problem 630
Consider the matrix $A=\begin{bmatrix}
a & -b\\
b& a
\end{bmatrix}$, where $a$ and $b$ are real numbers and $b\neq 0$.
(a) Find all eigenvalues of $A$.
(b) For each eigenvalue of $A$, determine the eigenspace $E_{\lambda}$.
(c) Diagonalize the matrix $A$ by finding a nonsingular matrix $S$ and a diagonal matrix $D$ such that $S^{-1}AS=D$.
Add to solve later Diagonalize a 2 by 2 Symmetric Matrix
Problem 629
Diagonalize the $2\times 2$ matrix $A=\begin{bmatrix}
2 & -1\\
-1& 2
\end{bmatrix}$ by finding a nonsingular matrix $S$ and a diagonal matrix $D$ such that $S^{-1}AS=D$.
Add to solve later Determine the Number of Elements of Order 3 in a Non-Cyclic Group of Order 57
Problem 628
Let $G$ be a group of order $57$. Assume that $G$ is not a cyclic group.
Then determine the number of elements in $G$ of order $3$.
Read solution
Add to solve later Is the Following Function $T:\R^2 \to \R^3$ a Linear Transformation?
Problem 627
Determine whether the function $T:\R^2 \to \R^3$ defined by
\[T\left(\, \begin{bmatrix}
x \\
y
\end{bmatrix} \,\right)
=
\begin{bmatrix}
x_+y \\
x+1 \\
3y
\end{bmatrix}\]
is a linear transformation.
Add to solve later If There are 28 Elements of Order 5, How Many Subgroups of Order 5?
Problem 626
Let $G$ be a group. Suppose that the number of elements in $G$ of order $5$ is $28$.
Determine the number of distinct subgroups of $G$ of order $5$.
Add to solve later Union of Two Subgroups is Not a Group
Problem 625
Let $G$ be a group and let $H_1, H_2$ be subgroups of $G$ such that $H_1 \not \subset H_2$ and $H_2 \not \subset H_1$.
(a) Prove that the union $H_1 \cup H_2$ is never a subgroup in $G$.
(b) Prove that a group cannot be written as the union of two proper subgroups.
Add to solve later Ring Homomorphisms and Radical Ideals
Problem 624
Let $R$ and $R’$ be commutative rings and let $f:R\to R’$ be a ring homomorphism.
Let $I$ and $I’$ be ideals of $R$ and $R’$, respectively.
(a) Prove that $f(\sqrt{I}\,) \subset \sqrt{f(I)}$.
(b) Prove that $\sqrt{f^{-1}(I’)}=f^{-1}(\sqrt{I’})$
(c) Suppose that $f$ is surjective and $\ker(f)\subset I$. Then prove that $f(\sqrt{I}\,) =\sqrt{f(I)}$
Add to solve later Example of an Element in the Product of Ideals that Cannot be Written as the Product of Two Elements
Problem 623
Let $I=(x, 2)$ and $J=(x, 3)$ be ideal in the ring $\Z[x]$.
(a) Prove that $IJ=(x, 6)$.
(b) Prove that the element $x\in IJ$ cannot be written as $x=f(x)g(x)$, where $f(x)\in I$ and $g(x)\in J$.
Add to solve later If the Sum of Entries in Each Row of a Matrix is Zero, then the Matrix is Singular
Problem 622
Let $A$ be an $n\times n$ matrix. Suppose that the sum of elements in each row of $A$ is zero.
Then prove that the matrix $A$ is singular.
Add to solve later Normal Subgroup Whose Order is Relatively Prime to Its Index
Problem 621
Let $G$ be a finite group and let $N$ be a normal subgroup of $G$.
Suppose that the order $n$ of $N$ is relatively prime to the index $|G:N|=m$.
(a) Prove that $N=\{a\in G \mid a^n=e\}$.
(b) Prove that $N=\{b^m \mid b\in G\}$.
Add to solve later 
