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# Constructing a Basis for a Vector Space

schedule Aug 12, 2023
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Linear Algebra
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Theorem.

# A set in n-dimensional vector space containing more or less than n vectors

Let $V$ be a vector space and let $\{\boldsymbol{v}_1,\boldsymbol{v}_2,\cdots,\boldsymbol{v}_n\}$ be a basis for $V$.

• If a set has more than $n$ vectors, then the set is linearly dependent.

• If a set has fewer than $n$ vectors, then the set does not span $V$.

Proof. We start by proving the first case. Suppose a set $W$ contains $m$ vectors in $V$ defined below:

$$W = \{\boldsymbol{w}_1,\boldsymbol{w}_2,\cdots,\boldsymbol{w}_m\}$$

Where $m\gt{n}$. To showlink that $W$ is linearly dependent, we must show that there exists a non-zero solution to:

$$$$\label{eq:YrWzjiXdbNK669rfWH4} x_1\boldsymbol{w}_1+ x_2\boldsymbol{w}_2+\cdots+ x_m\boldsymbol{w}_m=\boldsymbol{0}$$$$

Since $\{\boldsymbol{v}_1,\boldsymbol{v}_2,\cdots,\boldsymbol{v}_n\}$ are basis vectors for V, we know that all vectors in V can be expressed as a linear combination of these vectors. Since the vectors in W are in V, there must exist some:

\begin{align*} \boldsymbol{w}_1&=a_{1,1}\boldsymbol{v}_1\;+\;a_{1,2}\boldsymbol{v}_2+\dots+a_{1,n}\boldsymbol{v}_n\\ \boldsymbol{w}_2&=a_{2,1}\boldsymbol{v}_1\;+\;a_{2,2}\boldsymbol{v}_2+\dots+a_{2,n}\boldsymbol{v}_n\\ &\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\\ \boldsymbol{w}_m&=a_{m,1}\boldsymbol{v}_1+a_{m,2}\boldsymbol{v}_2+\dots+a_{m,n}\boldsymbol{v}_n \end{align*}

Consider the homogeneous system of linear equations:

\begin{aligned} a_{1,1}x_1 +\;a_{2,1}x_2 \;+\cdots+a_{m,1}x_m&=0\\ a_{1,2}x_1 +\;a_{2,2}x_2 \;+\cdots+a_{m,2}x_m&=0\\ \vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;&\vdots\\ a_{1,n}x_1 +\;a_{2,n}x_2 +\cdots+a_{m,n}x_m&=0 \end{aligned}

We know from theoremlink that because there are more unknowns than equations ($m\gt{n}$), the linear system has infinitely many non-zero solutions $x_1$, $x_2$, $\cdots$, $x_m$. Since there exists at least one non-zero solution to \eqref{eq:YrWzjiXdbNK669rfWH4}, we conclude that $W$ is linearly dependent.

* * *

Let us now prove the second case. Suppose $W$ is set of $m$ vectors in $V$ defined below:

$$W = \{\boldsymbol{w}_1,\boldsymbol{w}_2,\cdots,\boldsymbol{w}_m\}$$

Where $m\lt{n}$. Let's prove that $W$ does not span $V$ by contradiction. Consider the linear combination of $n$ basis vectors $\boldsymbol{v}_1$, $\boldsymbol{v}_2$, $\cdots$, $\boldsymbol{v}_n$ in $V$ below:

$$$$\label{eq:L3YPAqf2SZDnsdBkiDR} x_1\boldsymbol{v}_1+x_2\boldsymbol{v}_2+\cdots+x_n\boldsymbol{v}_n$$$$

Note that we did not specify that $V$ is $n$-dimensional here - we're simply stating that there are $n$ basis vectors in $V$. We will later prove that if $V$ has $n$ basis vectors, then $V$ must be $n$-dimensional. For now though, you can assume that $V$ could be any dimension.

Now, assume that $W$ spans $V$, which means that every vector in $V$ can be expressed as a linear combination of vectors in $W$, that is:

\label{eq:mw2Y6ey9JpHDhideRi4} \begin{aligned} \boldsymbol{v}_1&= a_{1,1}\boldsymbol{w}_1+a_{2,1}\boldsymbol{w}_2+\dots+a_{m,1}\boldsymbol{w}_m\\ \boldsymbol{v}_2&=a_{1,2}\boldsymbol{w}_1+a_{2,2}\boldsymbol{w}_2+\dots+a_{m,2}\boldsymbol{w}_m\\ &\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\\ \boldsymbol{v}_n&=a_{1,n}\boldsymbol{w}_1+a_{2,n}\boldsymbol{w}_2+\dots+a_{m,n}\boldsymbol{w}_m \end{aligned}

Now, suppose we are solving the following homogenous system of linear equations:

\label{eq:uZdCZy35fVid8nJn2xh} \begin{aligned} a_{1,1}x_1 \;+\;a_{1,2}x_2 \;+\cdots+a_{1,n}x_n&=0\\ a_{2,1}x_1 \;+\;a_{2,2}x_2 \;+\cdots+a_{2,n}x_n&=0\\ \vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;&\vdots\\ a_{m,1}x_1 +\;a_{m,2}x_2 +\cdots+a_{m,n}x_n&=0 \end{aligned}

Since $m\lt{n}$ in \eqref{eq:uZdCZy35fVid8nJn2xh}, there are more columns than rows (or more unknowns than equations), which means that the system \eqref{eq:uZdCZy35fVid8nJn2xh} has infinitely many non-zero solutions $x_1$, $x_2$, $\cdots$, $x_n$ by theoremlink.

Now, let's substitute the vectors $\boldsymbol{v}_1$, $\boldsymbol{v}_2$, $\cdots$, $\boldsymbol{v}_n$ from \eqref{eq:mw2Y6ey9JpHDhideRi4} into \eqref{eq:L3YPAqf2SZDnsdBkiDR} to get:

\label{eq:L0nggT3E0tYyhqM4HMW} \begin{aligned} &\;\;\;\;\;x_1\boldsymbol{v}_1+x_2\boldsymbol{v}_2+\cdots+x_n\boldsymbol{v}_n\\ &=x_1(a_{1,1}\boldsymbol{w}_1+a_{2,1}\boldsymbol{w}_2+\dots+a_{m,1}\boldsymbol{w}_m)\\ &+x_2(a_{1,2}\boldsymbol{w}_1+a_{2,2}\boldsymbol{w}_2+\dots+a_{m,2}\boldsymbol{w}_m)\\ &\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\\ &+x_n(a_{1,n}\boldsymbol{w}_1+a_{2,n}\boldsymbol{w}_2+\dots+a_{m,n}\boldsymbol{w}_m) \end{aligned}

We can reformulate \eqref{eq:L0nggT3E0tYyhqM4HMW} as:

\label{eq:KIQKJlzx5XlMM9MP2LK} \begin{aligned} &\;\;\;\;\;x_1\boldsymbol{v}_1+x_2\boldsymbol{v}_2+\cdots+x_n\boldsymbol{v}_n\\ &=w_1(a_{1,1}x_1+a_{1,2}x_2+\cdots+a_{1,n}x_n)\\ &+w_2(a_{2,1}x_1+a_{2,2}x_2+\cdots+a_{2,n}x_n)\\ &\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\vdots\\ &+w_m(a_{m,1}x_1+a_{m,2}x_2+\dots+a_{m,n}x_n) \end{aligned}

The sums in the brackets of \eqref{eq:KIQKJlzx5XlMM9MP2LK} are given by \eqref{eq:uZdCZy35fVid8nJn2xh}, which means:

\begin{align*} x_1\boldsymbol{v}_1+x_2\boldsymbol{v}_2+\cdots+x_n\boldsymbol{v}_n &=\boldsymbol{w}_1(0)+\boldsymbol{w}_2(0)+\cdots+\boldsymbol{w}_m(0)\\ &=\boldsymbol{0} \end{align*}

Let's summarize what we have derived:

$$x_1\boldsymbol{v}_1+x_2\boldsymbol{v}_2+\cdots+x_n\boldsymbol{v}_n=\boldsymbol{0}$$

We have already shown that there exists at least one non-zero solution $x_1$, $x_2$, $\cdots$, $x_n$. Therefore, by definitionlink of linear dependence, we have that $\boldsymbol{v}_1$, $\boldsymbol{v}_2$, $\cdots$, $\boldsymbol{v}_n$ are linearly dependent. This contradicts the fact that $\boldsymbol{v}_1$, $\boldsymbol{v}_2$, $\cdots$, $\boldsymbol{v}_n$ are linearly independent as they are basis vectors. Since we have a contradiction, our assumption that $W$ spans $V$ is incorrect. This proves that $W$ does not span $V$.

Theorem.

# Number of bases for a finite-dimensional vector space

All bases for a finite-dimensional vector space have the same number of vectors.

Proof. Consider an $n$-dimensional vector space $V$. In theoremlink, we have shown that:

• if a set has more than $n$ vectors, then the set is linearly dependent.

• if a set has fewer than $n$ vectors, then the set does not span $V$.

By definition, basis vectors must:

• be linearly independent.

• span the vector space $V$.

For these properties to be satisfied, there must be exactly $n$ basis vectors. This completes the proof.

Theorem.

# Number of basis vectors of an n-dimensional vector space

The number of basis vectors in an $n$-dimensional vector space is exactly $n$. In other words, every basis of $\mathbb{R}^n$ contains exactly $n$ vectors.

Proof. We know from this section that the standard basis for $\mathbb{R}^n$ is:

$$\left\{ \begin{pmatrix} 1\\0\\\vdots\\0 \end{pmatrix}, \begin{pmatrix} 0\\1\\\vdots\\0 \end{pmatrix},\;\cdots\;, \begin{pmatrix} 0\\0\\\vdots\\1 \end{pmatrix} \right\}$$

Here, there are $n$ vectors in this basis. From theorem, we know that all bases for a finite-dimensional vector space have the same number of vectors. Since the standard basis for $\mathbb{R}^n$ has n vectors, we conclude that all bases of $\mathbb{R}^n$ must also have exactly $n$ vectors.

Using this theorem, we can now add one slight detail to theorem - instead of any arbitrary vector space $V$, we now know that $V$ must be $n$-dimensional if $V$ has $n$ basis vectors.

Theorem.

# Relationship between dimensionality of vector space and linear dependence and span

Let $V$ be an $n$-dimensional vector space.

• If a set has more than $n$ vectors, then the set is linearly dependent.

• If a set has fewer than $n$ vectors, then the set does not span $V$.

Example.

## Showing that a set of vectors is linearly dependent by inspection

Consider a set $S$ containing the following vectors:

$$\boldsymbol{v}_1= \begin{pmatrix} 3\\5 \end{pmatrix},\;\;\;\;\; \boldsymbol{v}_2=\begin{pmatrix} 2\\1 \end{pmatrix},\;\;\;\;\; \boldsymbol{v}_3=\begin{pmatrix} 4\\3 \end{pmatrix}$$

Is $S$ linearly dependent or independent?

Solution. These vectors reside in $\mathbb{R}^2$. Because this set contains more than $2$ vectors, we know by theoremlink that the set is linear dependent. This also means that the set is not a basis for $\mathbb{R}^2$.

Example.

## Showing that a set of vectors does not span a vector space by inspection

Consider a set $S$ containing the following vector:

$$\boldsymbol{v}_1= \begin{pmatrix} 3\\5\\1 \end{pmatrix},\;\;\;\;\; \boldsymbol{v}_2=\begin{pmatrix} 2\\1\\2 \end{pmatrix}$$

Give a reason why $S$ does not span $\mathbb{R}^3$.

Solution. By theoremlink, we know that $2$ vectors cannot span $\mathbb{R}^3$. This also means that $S$ is not a basis for $\mathbb{R}^3$.

Definition.

# Dimension of a vector space

The dimension of a finite-dimensional vector space $V$, denoted by $\mathrm{dim}(V)$, is the number of basis vectors for $V$.

Example.

## Finding the dimension of Rn

Find the dimension of $\mathbb{R}^2$ and $\mathbb{R}^n$.

Solution. Because $\mathbb{R}^2$ has $2$ basis vectors, its dimension is $2$, that is, $\dim(\mathbb{R}^2)=2$. Similarly, $\dim(\mathbb{R}^n)=n$.

Example.

## Finding the dimension of a spanning set

Consider the following spanning set:

$$H=\text{span}(\boldsymbol{v}_1,\boldsymbol{v}_2)$$

Where vectors $\boldsymbol{v}_1$ and $\boldsymbol{v}_2$ are:

$$\boldsymbol{v}_1= \begin{pmatrix} 3\\5\\1 \end{pmatrix},\;\;\;\;\; \boldsymbol{v}_2=\begin{pmatrix} 2\\1\\4 \end{pmatrix}$$

Find the dimension of $H$.

Solution. To find the dimension of $H$, we first must find the basis for $H$. The two vectors $\boldsymbol{v}_1$ and $\boldsymbol{v}_2$ are linearly independent and $H$ is defined to be their span. By definition then, the vectors form a basis for $H$. The basis is therefore:

$$\mathrm{Basis}=\{\boldsymbol{v}_1,\boldsymbol{v}_2\}$$

The dimension is simply the number of vectors in our basis, which means $\text{dim}(H)=2$. This should make sense because all possible linear combinations of $\boldsymbol{v}_1$ and $\boldsymbol{v}_2$ will form a plane, which is a two-dimensional entity.

Theorem.

# Plus/Minus theorem

Let $S$ be a non-empty set of vectors in vector space $V$.

• Let $S$ span $V$. If we remove a vector from $S$ that is linearly dependent on the other vectors of $S$, then $S$ still spans $V$.

• Let $S$ be linearly independent. If we insert a new vector that is outside the span of $S$ into $S$, then $S$ will still remain linearly independent.

Proof. We start by proving the first case. Let $S$ span $V$. Suppose there exists a vector $\boldsymbol{v}$ in $S$ that is linearly dependent on the other vectors of $S$, that is, $\boldsymbol{v}$ can be expressed as a linear combination of vectors in $S$. Removing $\boldsymbol{v}$ does not affect the span of $S$ because $\boldsymbol{v}$ can still be constructed using a linear combination of the remaining vectors in $S$.

Next, we prove the second case. Let $S$ be linearly independent. Suppose there exists a vector $\boldsymbol{v}$ in $V$ such that $\boldsymbol{v}$ is outside the span of $S$. By definition of span, this means that $\boldsymbol{v}$ cannot be constructed using a linear combination of vectors in $V$. Therefore, inserting $\boldsymbol{v}$ into $S$ will keep $S$ linearly independent.

This completes the proof.

Theorem.

# Constructing a basis for a vector space

Let $S$ be a finite set of vectors in a finite dimensional vector space $V$.

• If $S$ spans $V$ but is not a basis for $V$, then $S$ can be reduced to a basis for $V$.

• If $S$ is a linearly independent set but is not a basis for $V$, then $S$ can be expanded to a basis for $V$.

Proof. Let's start by proving the first case. If $S$ spans $V$ but is not a basis of $V$, then by definition of basis, this means that there exists at least one pair of linearly dependent vectors. We remove one of these vectors from $S$. From the plus/minus theoremlink, we know that this new set $S$ still spans vector $V$ because removing a linearly dependent vector from a spanning set does not affect the span. We repeatedly remove any linearly dependent vector from $S$ until we are left with a set of linearly independent vectors that spans $V$ - at this point, $S$ becomes a basis for $V$ by definition.

Next, let's prove the second case. If $S$ is a linearly independent set with $k$ vectors $\boldsymbol{v}_1$, $\boldsymbol{v}_2$, $\cdots$, $\boldsymbol{v}_k$ but is not a basis vector for $V$, then this means that $S$ does not span $V$ by the definition of basis. This means that there exists some $\boldsymbol{v}_{k+1}$ in $V$ that is not in $\mathrm{span}(S)$, that is, $\boldsymbol{v}_{k+1}$ cannot be constructed using the vectors in $S$. From the plus/minus theoremlink, we know that if we insert this vector $\boldsymbol{v}_{k+1}$ into $S$, then $\{\boldsymbol{v}_1,\boldsymbol{v}_2,\cdots,\boldsymbol{v}_{k},\boldsymbol{v}_{k+1}\}$ will still be linearly independent. We continue expanding $S$ in this way until $S$ spans $V$, thereby making $S$ a basis for $V$.

This completes the proof.

Theorem.

# Triangular relationship between basis vectors, span and linear dependence

Let $V$ be an $n$-dimensional vector space and let $S$ be a set of $n$ vectors in $V$. Then $S$ is a basis for $V$ if and only if either:

• $S$ spans $V$.

• or $S$ is linearly independent.

Proof. Let's prove the forward proposition that if $S$ is a basis consisting of $n$ vectors in an $n$-dimensional vector space, then $S$ spans $V$ and $S$ is linearly independent. There is actually nothing to prove here because by definition of basis, $S$ is a linearly independent set that also spans $V$.

Let's now prove the backward proposition that if $S$ is a set of $n$ vectors in an $n$-dimensional vector space $V$ and $S$ spans $V$, then $S$ is a basis for $V$. Since we are given that $S$ spans $V$, we need to show that $S$ is also a linearly independent set. Let's assume the contrary that $S$ is a linearly dependent set. This means that there exists at least one vector that can be expressed as a linear combination of the other vectors in $S$. By the plus/minus theoremlink, we can remove this vector and the remaining $n-1$ vectors must also span $V$. However, this cannot be true because by theoremlink, if a set has fewer than $n$ vectors, then the set does not span $V$. We have a contradiction here, which means that our initial assumption that $S$ is a linearly dependent set is false. In other words, $S$ must be linearly independent. Because $S$ spans $V$ and is also a linearly independent set, then this means that $S$ must be a basis for $V$ by definition.

Next, let's prove that if $S$ is a set of $n$ linearly independent vectors in an $n$-dimensional vector space $V$, then $S$ is a basis for $V$. Since we are given that $S$ is linearly independent, we need to show that $S$ spans $V$. Let's assume the contrary once again that $S$ does not span $V$. This means that there exists at least one vector in $V$ that resides outside the span of $S$, that is, at least one vector in $V$ cannot be expressed as a linear combination of vectors in $S$. By the plus/minus theoremlink, we can add this vector from $S$ while keeping $S$ a linearly independent set. We now have $n+1$ linearly independent vectors in an $n$-dimensional vector space $V$, which is impossible because by theoremlink, if a set has more than $n$ vectors, then the set must be linearly dependent. We have a contradiction here, which means that our assumption that $S$ does not span $V$ is false. Because $S$ is a linearly independent set that spans $V$, we conclude that $S$ must be a basis for $V$ by definition.

This completes the proof.

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