## Subspace Spanned by Trigonometric Functions $\sin^2(x)$ and $\cos^2(x)$

## Problem 612

Let $C[-2\pi, 2\pi]$ be the vector space of all real-valued continuous functions defined on the interval $[-2\pi, 2\pi]$.

Consider the subspace $W=\Span\{\sin^2(x), \cos^2(x)\}$ spanned by functions $\sin^2(x)$ and $\cos^2(x)$.

**(a)** Prove that the set $B=\{\sin^2(x), \cos^2(x)\}$ is a basis for $W$.

**(b)** Prove that the set $\{\sin^2(x)-\cos^2(x), 1\}$ is a basis for $W$.

## Is the Set of All Orthogonal Matrices a Vector Space?

## Problem 611

An $n\times n$ matrix $A$ is called **orthogonal** if $A^{\trans}A=I$.

Let $V$ be the vector space of all real $2\times 2$ matrices.

Consider the subset

\[W:=\{A\in V \mid \text{$A$ is an orthogonal matrix}\}.\]
Prove or disprove that $W$ is a subspace of $V$.

## Linear Transformation $T:\R^2 \to \R^2$ Given in Figure

## Problem 610

Let $T:\R^2\to \R^2$ be a linear transformation such that it maps the vectors $\mathbf{v}_1, \mathbf{v}_2$ as indicated in the figure below.

Find the matrix representation $A$ of the linear transformation $T$.

Add to solve later## Eigenvalues of $2\times 2$ Symmetric Matrices are Real by Considering Characteristic Polynomials

## Problem 609

Let $A$ be a $2\times 2$ real symmetric matrix.

Prove that all the eigenvalues of $A$ are real numbers by considering the characteristic polynomial of $A$.

## If Matrices Commute $AB=BA$, then They Share a Common Eigenvector

## Problem 608

Let $A$ and $B$ be $n\times n$ matrices and assume that they commute: $AB=BA$.

Then prove that the matrices $A$ and $B$ share at least one common eigenvector.

## Find a Basis of the Subspace Spanned by Four Polynomials of Degree 3 or Less

## Problem 607

Let $\calP_3$ be the vector space of all polynomials of degree $3$ or less.

Let

\[S=\{p_1(x), p_2(x), p_3(x), p_4(x)\},\]
where

\begin{align*}

p_1(x)&=1+3x+2x^2-x^3 & p_2(x)&=x+x^3\\

p_3(x)&=x+x^2-x^3 & p_4(x)&=3+8x+8x^3.

\end{align*}

**(a)** Find a basis $Q$ of the span $\Span(S)$ consisting of polynomials in $S$.

**(b)** For each polynomial in $S$ that is not in $Q$, find the coordinate vector with respect to the basis $Q$.

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

Read solution

## Determine the Dimension of a Mysterious Vector Space From Coordinate Vectors

## Problem 606

Let $V$ be a vector space and $B$ be a basis for $V$.

Let $\mathbf{w}_1, \mathbf{w}_2, \mathbf{w}_3, \mathbf{w}_4, \mathbf{w}_5$ be vectors in $V$.

Suppose that $A$ is the matrix whose columns are the coordinate vectors of $\mathbf{w}_1, \mathbf{w}_2, \mathbf{w}_3, \mathbf{w}_4, \mathbf{w}_5$ with respect to the basis $B$.

After applying the elementary row operations to $A$, we obtain the following matrix in reduced row echelon form

\[\begin{bmatrix}

1 & 0 & 2 & 1 & 0 \\

0 & 1 & 3 & 0 & 1 \\

0 & 0 & 0 & 0 & 0 \\

0 & 0 & 0 & 0 & 0

\end{bmatrix}.\]

**(a)** What is the dimension of $V$?

**(b)** What is the dimension of $\Span\{\mathbf{w}_1, \mathbf{w}_2, \mathbf{w}_3, \mathbf{w}_4, \mathbf{w}_5\}$?

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

Read solution

## Matrix Representation, Rank, and Nullity of a Linear Transformation $T:\R^2\to \R^3$

## Problem 605

Let $T:\R^2 \to \R^3$ be a linear transformation such that

\[T\left(\, \begin{bmatrix}

3 \\

2

\end{bmatrix} \,\right)

=\begin{bmatrix}

1 \\

2 \\

3

\end{bmatrix} \text{ and }

T\left(\, \begin{bmatrix}

4\\

3

\end{bmatrix} \,\right)

=\begin{bmatrix}

0 \\

-5 \\

1

\end{bmatrix}.\]

**(a)** Find the matrix representation of $T$ (with respect to the standard basis for $\R^2$).

**(b)** Determine the rank and nullity of $T$.

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

Read solution

## Find Bases for the Null Space, Range, and the Row Space of a $5\times 4$ Matrix

## Problem 604

Let

\[A=\begin{bmatrix}

1 & -1 & 0 & 0 \\

0 &1 & 1 & 1 \\

1 & -1 & 0 & 0 \\

0 & 2 & 2 & 2\\

0 & 0 & 0 & 0

\end{bmatrix}.\]

**(a)** Find a basis for the null space $\calN(A)$.

**(b)** Find a basis of the range $\calR(A)$.

**(c)** Find a basis of the row space for $A$.

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

Read solution

## Are the Trigonometric Functions $\sin^2(x)$ and $\cos^2(x)$ Linearly Independent?

## Problem 603

Let $C[-2\pi, 2\pi]$ be the vector space of all continuous functions defined on the interval $[-2\pi, 2\pi]$.

Consider the functions \[f(x)=\sin^2(x) \text{ and } g(x)=\cos^2(x)\]
in $C[-2\pi, 2\pi]$.

Prove or disprove that the functions $f(x)$ and $g(x)$ are linearly independent.

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

Read solution

## Find an Orthonormal Basis of the Given Two Dimensional Vector Space

## Problem 602

Let $W$ be a subspace of $\R^4$ with a basis

\[\left\{\, \begin{bmatrix}

1 \\

0 \\

1 \\

1

\end{bmatrix}, \begin{bmatrix}

0 \\

1 \\

1 \\

1

\end{bmatrix} \,\right\}.\]

Find an ortho**normal** basis of $W$.

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

Read solution

## Vector Space of 2 by 2 Traceless Matrices

## Problem 601

Let $V$ be the vector space of all $2\times 2$ matrices whose entries are real numbers.

Let

\[W=\left\{\, A\in V \quad \middle | \quad A=\begin{bmatrix}

a & b\\

c& -a

\end{bmatrix} \text{ for any } a, b, c\in \R \,\right\}.\]

**(a)** Show that $W$ is a subspace of $V$.

**(b)** Find a basis of $W$.

**(c)** Find the dimension of $W$.

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

Read solution

## Find an Orthonormal Basis of $\R^3$ Containing a Given Vector

## Problem 600

Let $\mathbf{v}_1=\begin{bmatrix}

2/3 \\ 2/3 \\ 1/3

\end{bmatrix}$ be a vector in $\R^3$.

Find an orthonormal basis for $\R^3$ containing the vector $\mathbf{v}_1$.

Add to solve later## The Inverse Matrix of a Symmetric Matrix whose Diagonal Entries are All Positive

## Problem 599

Let $A$ be a real symmetric matrix whose diagonal entries are all positive real numbers.

Is it true that the all of the diagonal entries of the inverse matrix $A^{-1}$ are also positive?

If so, prove it. Otherwise, give a counterexample.

## If Every Proper Ideal of a Commutative Ring is a Prime Ideal, then It is a Field.

## Problem 598

Let $R$ be a commutative ring with $1$.

Prove that if every proper ideal of $R$ is a prime ideal, then $R$ is a field.

Add to solve later## Linear Transformation that Maps Each Vector to Its Reflection with Respect to $x$-Axis

## Problem 597

Let $F:\R^2\to \R^2$ be the function that maps each vector in $\R^2$ to its reflection with respect to $x$-axis.

Determine the formula for the function $F$ and prove that $F$ is a linear transformation.

Add to solve later## An Example of a Real Matrix that Does Not Have Real Eigenvalues

## Problem 596

Let

\[A=\begin{bmatrix}

a & b\\

-b& a

\end{bmatrix}\]
be a $2\times 2$ matrix, where $a, b$ are real numbers.

Suppose that $b\neq 0$.

Prove that the matrix $A$ does not have real eigenvalues.

Add to solve later## The Intersection of Two Subspaces is also a Subspace

## Problem 595

Let $U$ and $V$ be subspaces of the $n$-dimensional vector space $\R^n$.

Prove that the intersection $U\cap V$ is also a subspace of $\R^n$.

Add to solve later## Example of an Infinite Group Whose Elements Have Finite Orders

## Problem 594

Is it possible that each element of an infinite group has a finite order?

If so, give an example. Otherwise, prove the non-existence of such a group.

## Eigenvalues and Eigenvectors of The Cross Product Linear Transformation

## Problem 593

We fix a nonzero vector $\mathbf{a}$ in $\R^3$ and define a map $T:\R^3\to \R^3$ by

\[T(\mathbf{v})=\mathbf{a}\times \mathbf{v}\]
for all $\mathbf{v}\in \R^3$.

Here the right-hand side is the cross product of $\mathbf{a}$ and $\mathbf{v}$.

**(a)** Prove that $T:\R^3\to \R^3$ is a linear transformation.

**(b)** Determine the eigenvalues and eigenvectors of $T$.