# Quadratic Form

Nov 4, 2020
15 min read
May 22, 2021 06:37 UTC
This long post covers the quadratic form and the positive definiteness of matrices. The decomposition of symmetric matrices is slightly touched on, and the entire post is mainly to prepare for the next chapter -- eigenvalues and eigenvectors.

The quadratic form of a matrix is a function of some vector. We’re going to discuss several cases, starting with the simplest case of a linear form.

## Definitions

### Linear form

The linear form of $\boldsymbol{x} = (x_1, \cdots, x_n)'$ is

$$\begin{gathered} \boldsymbol{a}'\boldsymbol{x} = a_1 x_1 + a_2 x_2 + \cdots + a_n x_n \\ \boldsymbol{Ax} = \begin{pmatrix} \boldsymbol{\alpha}_1'\boldsymbol{x} \\ \vdots \\ \boldsymbol{\alpha}_m'\boldsymbol{x} \end{pmatrix} \end{gathered}$$

where $\boldsymbol{\alpha}_i'$ denotes the $i$-th row of $\boldsymbol{A}$. Each row of $\boldsymbol{Ax}$ is a linear combination of the elements of $\boldsymbol{x}$, which is why it is called a linear functional or linear form. Note that $\boldsymbol{A}$ is a given matrix, and $\boldsymbol{x}$ is just a vector with known dimensions1.

### Bilinear form

Taking this a step further, we now have two vectors $\boldsymbol{x}$ and $\boldsymbol{y}$:

$$\boldsymbol{x}'\boldsymbol{Ay}, \quad \boldsymbol{A}: m \times n, \boldsymbol{x}: m \times 1, \boldsymbol{y}: n \times 1$$

We’re multiplying a row vector to a matrix, then two a column vector, so the outcome is going to be a scalar value. Again, only $\boldsymbol{A}$ is a known matrix, and the vectors $\boldsymbol{x}$ and $\boldsymbol{y}$ are just unknowns.

\begin{aligned} \boldsymbol{x}'\boldsymbol{Ay} &= (x_1, \cdots, x_m) \begin{pmatrix} a_{11} & \cdots & a_{1n} \\ \vdots & \ddots & \vdots \\ a_{m1} & \cdots & a_{mn} \end{pmatrix} \begin{pmatrix} y_1 \\ \vdots \\ y_n \end{pmatrix} \\ &= \sum_{j=1}^n \sum_{i=1}^m a_{ij}x_i y_j \end{aligned}

For example, let

$$\boldsymbol{A} = \begin{pmatrix} 2 & 1 & 3 \\ -1 & 0 & 1 \end{pmatrix}, \quad \boldsymbol{x}'\boldsymbol{Ay} = 2x_1y_1 + x_1y_2 + 3x_1y_3 - x_2y_1 + x_2y_3$$

For some fixed $\boldsymbol{x}$, it becomes a linear function of $\boldsymbol{y}$. Similarly if $\boldsymbol{y}$ is fixed, then it becomes a linear function of $\boldsymbol{x}$.

### Quadratic form

The difference the bilinear form and the quadratic form is that now $\boldsymbol{x}$ and $\boldsymbol{y}$ are equal to each other, and thus $\boldsymbol{A}$ must be a square matrix.

Any $n \times n$ real matrix $\boldsymbol{A}$ determines a quadratic form in $n$ variables $\boldsymbol{x} = (x_1, \cdots, x_n)'$ by the formula

$$\boldsymbol{x}'\boldsymbol{Ax} = \sum_{j=1}^n \sum_{i=1}^n a_{ij}x_i x_j = \sum_{i=1}^n a_{ii}x_i^2 + \sum_{i \neq j} a_{ij}x_i x_j$$

For example, let

$$\boldsymbol{A} = \begin{pmatrix} 1 & 2 \\ 3 & 4 \end{pmatrix}$$

The quadratic form is then

$$\boldsymbol{x}'\boldsymbol{Ax} = x_1^2 + 4x_2^2 + 2x_1x_2 + 3x_2x_1 = x_1^2 + 4x_2^2 + 5x_1x_2$$

So why are we doing this? The point is that investigating properties of this function reveals properties about the matrix itself. Knowing the function $x_1^2 + 4x_2^2 + 5x_1x_2$ in the space $\mathbb{R}^2$ tells us a lot about $\boldsymbol{A}$.

### Notes on the quadratic form

1. $\boldsymbol{x}'\boldsymbol{Ax} = \boldsymbol{x}'\boldsymbol{A}'\boldsymbol{x}$. The LHS $\boldsymbol{x}'\boldsymbol{Ax}$ is a scalar and thus symmetric, so

$$\left(\boldsymbol{x}'\boldsymbol{Ax}\right)' = \boldsymbol{x}'\boldsymbol{A}'\boldsymbol{x} = RHS$$

This implies that as far as quadratic forms are concerned, we can confine to symmetric matrices, i.e. for any $n \times n$ matrix $\boldsymbol{A}$, there exists a symmetric matrix $\boldsymbol{B}$ that has exactly the same quadratic forms.

$$\boldsymbol{B} = \frac{\boldsymbol{A} + \boldsymbol{A}'}{2}, \quad \boldsymbol{x}'\boldsymbol{Bx} = \frac{1}{2}\boldsymbol{x}'\boldsymbol{Ax} + \frac{1}{2} \boldsymbol{x}'\boldsymbol{A}'\boldsymbol{x} = \boldsymbol{x}'\boldsymbol{Ax}$$

2. If $\boldsymbol{A}$ and $\boldsymbol{B}$ are symmetric matrices, then

$$\boldsymbol{x}'\boldsymbol{Ax} = \boldsymbol{x}'\boldsymbol{Bx} \Longleftrightarrow \boldsymbol{A} = \boldsymbol{B}$$

## Categorizing quadratic forms

Now we’re ready to talk about categorizing matrices, or equivalently categorizing quadratic forms. The concepts we’re going to introduce come from a desire for notions about the signs of matrices. In elementary maths, we have $3 > 0$ and $-3 < 0$, but for two matrices:

$$\boldsymbol{A} = \begin{pmatrix} 1 & 2 \\ 2 & 3 \end{pmatrix} \quad vs. \quad \boldsymbol{B} = \begin{pmatrix} 2 & 1 \\ 1 & 2 \end{pmatrix},$$

we can’t easily say one is greater than the other. The notion of definiteness will help us with this comparison. Conceptually, we have

$$\boldsymbol{A} - \boldsymbol{B} = \begin{pmatrix} -1 & 1 \\ 1 & 1 \end{pmatrix},$$

and then we determine the definiteness of the matrix $\boldsymbol{A} - \boldsymbol{B}$ as a whole, and say “$\boldsymbol{A}$ is larger than $\boldsymbol{B}$” or the other way around.

### Non-negative definite

The definition of non-negative definite (NND) matrices applies to quadratic forms as well as to matrices directly. A quadratic form $\boldsymbol{x}'\boldsymbol{Ax}$ is NND (equivalently $\boldsymbol{A}$ is NND) if $\boldsymbol{x}'\boldsymbol{Ax} \geq 0$ for any $\boldsymbol{x} \in \mathbb{R}^n$.

Clearly, $\boldsymbol{x} = 0$ yields $\boldsymbol{x}'\boldsymbol{Ax} = 0$. Two distinct cases for NND are:

1. $\boldsymbol{x} = \boldsymbol{0}$ is the only vector that yields zero. This case is called positive definite (PD).
2. There are some $\boldsymbol{x} \neq \boldsymbol{0}$ that yield $\boldsymbol{x}'\boldsymbol{Ax} = 0$. This case is called positive semi-definite (PSD).

#### Examples

• The identity matrix is positive definite.

$$\boldsymbol{x}'\boldsymbol{Ix} = x_1^2 + x_2^2 + \cdots + x_n^2 \geq 0$$

• The matrix of ones is positive semi-definite.

\begin{aligned} \boldsymbol{x}'\boldsymbol{Jx} &= x_1^2 + \cdots + x_n^2 + 2(x_1 x_2 + \cdots + x_{n-1}x_n) \\ &= (x_1 + \cdots + x_n)^2 \geq 0 \quad \Rightarrow NND \end{aligned}

This can be zero if $x_1 + \cdots + x_n = 0$, so $\boldsymbol{J}$ is not positive-definite, and is thus positive semi-definite.

### Non-positive definite

On the opposite side of NND matrices, we have non-positive definite (NPD) matrices. We say a quadratic form $\boldsymbol{x}'\boldsymbol{Ax}$ is non-positive definite if $\boldsymbol{x}'\boldsymbol{Ax} \leq 0$. The two possible cases are:

1. Negative definite: $\boldsymbol{x} = \boldsymbol{0}$ is the only vector to make $\boldsymbol{x}'\boldsymbol{Ax} = 0$.
2. Negative semi-definite: similar to PSD but opposite.

### Indefinite

Given a real number $x$, if we say $x$ is not non-negative, then $x$ is negative. However, things are different for matrices. If a matrix is not NND, we can’t safely say it’s NPD. There exists a big “gray area” in the middle for matrices that are neither NND nor NPD called indefinite matrices. For those, $\boldsymbol{x}'\boldsymbol{Ax}$ can be negative and positive depending on $\boldsymbol{x}$.

## Properties

1. If $\boldsymbol{A}$ is NND/PD/PSD, then $-\boldsymbol{A}$ is NPD/ND/NSD, respectively.

2. If $\boldsymbol{D}$ is a diagonal matrix $\boldsymbol{D} = diag(d_1, \cdots, d_n)$, then

$$d_i \geq 0 \, \forall i \Longleftrightarrow \boldsymbol{D} \text{ is NND}$$

This is because

$$\boldsymbol{x}'\boldsymbol{Dx} = d_1x_1^2 + d_2x_2^2 + \cdots + d_nx_n^2 \geq 0$$

Following the same logic, we have

$$d_i > 0 \, \forall i \Longleftrightarrow \boldsymbol{D} \text{ is PD}$$

3. Multiplying a positive scalar to $\boldsymbol{A}$ does not change the definiteness.

4. If $\boldsymbol{A}$ and $\boldsymbol{B}$ are both NND, then $\boldsymbol{A} + \boldsymbol{B}$ is NND2.

$$\boldsymbol{x}'(\boldsymbol{A} + \boldsymbol{B})\boldsymbol{x} = \boldsymbol{x}'\boldsymbol{Ax} + \boldsymbol{x}'\boldsymbol{Bx} \geq 0$$

Similarly if $\boldsymbol{A}$ and $\boldsymbol{B}$ are both PD, then $\boldsymbol{A} + \boldsymbol{B}$ is PD.

However, if $\boldsymbol{A}$ and $\boldsymbol{B}$ are both PSD, $\boldsymbol{A} + \boldsymbol{B}$ is not necessarily PSD. $\boldsymbol{A} + \boldsymbol{B}$ is NND, but it could be PD or PSD.

Finally, if $\boldsymbol{A}$ is PSD and $\boldsymbol{B}$ is PD, then $\boldsymbol{A} + \boldsymbol{B}$ is PD.

1. A PD matrix is non-singular. An important corollary of this is that a PSD matrix is singular. This is one of the reasons why we put an emphasis on differentiating these two cases – singularity comes with a lot of properties!

To prove this, suppose $\boldsymbol{A}$ is singular. This means that the nullspace of $\boldsymbol{A}$ is not empty, i.e. there exists $\boldsymbol{x} \neq \boldsymbol{0}$ such that $\boldsymbol{x} \in \mathcal{N}(\boldsymbol{A})$. We then have

$$\boldsymbol{Ax} = \boldsymbol{0} \Rightarrow \boldsymbol{x}'\boldsymbol{Ax} = 0$$

We have found a non-zero $\boldsymbol{x}$ that can make the quadratic form zero, which is contradictory to “$\boldsymbol{A}$ is PD”.

2. Suppose $\boldsymbol{A}: n \times n$ and $\boldsymbol{P}: n \times m$.

1. If $\boldsymbol{A}$ is NND, then $\boldsymbol{P}'\boldsymbol{AP}$ is NND. This can be easily proven using the quadratic form:

$$\boldsymbol{x}'\boldsymbol{P}'\boldsymbol{APx} = (\boldsymbol{Px})'\boldsymbol{A}(\boldsymbol{Px}) \Rightarrow \boldsymbol{y}'\boldsymbol{Ay} \geq 0$$

2. If $\boldsymbol{A}$ is PD and $r(\boldsymbol{P}) = m$, then $\boldsymbol{P}'\boldsymbol{AP}$ is PD.

To show this, consider when $\boldsymbol{x}'\boldsymbol{P}'\boldsymbol{APx} = 0$. The fact that $\boldsymbol{A}$ is PD leads to $\boldsymbol{Px} = \boldsymbol{0}$. Since $r(\boldsymbol{P}) = m$, the $m \times m$ matrix $\boldsymbol{P}'\boldsymbol{P}$ is non-singular, and $\boldsymbol{P}'\boldsymbol{Px} = \boldsymbol{0}_m$ leads to $\boldsymbol{x} = \boldsymbol{0}$.

Similarly we can show that if $\boldsymbol{A}$ is PSD and $r(\boldsymbol{P}) = m$, then $\boldsymbol{P}'\boldsymbol{AP}$ is PSD.

3. If $\boldsymbol{A}$ is NND and $r(\boldsymbol{P}) < m$, then $\boldsymbol{P}'\boldsymbol{AP}$ is PSD. From the properties of matrix rank we know that $r(\boldsymbol{P}'\boldsymbol{AP}) < m$, so it’s singular and can’t be PD.

3. Suppose $\boldsymbol{A}: n \times n$ and $\boldsymbol{P}: n \times n$ and non-singular.

1. From (6) we know if $\boldsymbol{A}$ is PD, then $\boldsymbol{P}'\boldsymbol{AP}$ is PD.
2. If $\boldsymbol{A}$ is PSD, then $\boldsymbol{P}'\boldsymbol{AP}$ is PSD.
4. If $\boldsymbol{A}$ is PD, then $\boldsymbol{A}^{-1}$ is PD.

To prove this, it’s sufficient to show that $(\boldsymbol{A}^{-1})'$ is PD.

$$(\boldsymbol{A}^{-1})' = (\boldsymbol{A}^{-1})'\boldsymbol{A}\boldsymbol{A}^{-1} = \boldsymbol{P}'\boldsymbol{AP}$$

where $\boldsymbol{P} = \boldsymbol{A}^{-1}$. From (7), we have $\boldsymbol{P}'\boldsymbol{AP}$ is PD, so $(\boldsymbol{A}^{-1})'$ is PD.

5. Any principal submatrix of a PD (NND) matrix is PD (NND). A principal submatrix starts from the top-left corner of a square matrix, and is a square submatrix obtained by removing certain rows and columns.

To show this, let

$$\boldsymbol{P}_{n \times m} = \begin{pmatrix} \boldsymbol{I}_m \\ \boldsymbol{0}_{(n-m) \times m} \end{pmatrix}$$

and the rank of $\boldsymbol{P}$ is clearly $m$. $\boldsymbol{P}'\boldsymbol{AP}$ is the $m \times m$ principal submatrix of $\boldsymbol{A}$:

$$\begin{pmatrix} 1 & 0 \\ 0 & 1 \\ 0 & 0 \end{pmatrix}' \begin{pmatrix} 3 & 1 & 2 \\ 1 & 2 & 6 \\ 2 & 6 & 4 \end{pmatrix} \begin{pmatrix} 1 & 0 \\ 0 & 1 \\ 0 & 0 \end{pmatrix} = \begin{pmatrix} 3 & 1 \\ 1 & 2 \end{pmatrix}$$

Use (6) to see that $\boldsymbol{P}'\boldsymbol{AP}$ is PD.

This can be used to prove certain matrices are not PD. For example,

$$\boldsymbol{A} = \begin{pmatrix} -1 & 0 & 3 & 12 \\ 0 & -1 & 7 & 2 \\ \vdots & \vdots & \ddots & \vdots \end{pmatrix}$$

cannot be a positive definite matrix because the top-left submatrix is $-\boldsymbol{I}_{2}$, which is negative definite.

6. Any diagonal elements of a PD (NND) matrix is positive (non-negative):

$$\begin{gathered} \boldsymbol{A}: PD \Rightarrow a_{ii} > 0 \,\forall i \\ \boldsymbol{A}: NND \Rightarrow a_{ii} \geq 0 \,\forall i \end{gathered}$$

Note that this doesn’t go from right to left. To show this, let $\boldsymbol{p} = (0, \cdots, 0, 1, 0, \cdots, 0)'$ where only the $i$-th element is 1 and all the other $(n-1)$ elements are 0. $a_{ii} = \boldsymbol{p}'\boldsymbol{Ap}$ is positive definite, i.e. $a_{ii} > 0$.

Corollary of this, the trace of a PD matrix is positive, and the trace of an NND matrix is non-negative.

7. Let $\boldsymbol{P}: n \times m$ be any rectangular matrix. Then, $\boldsymbol{P}'\boldsymbol{P}$ and $\boldsymbol{PP}'$ are both non-negative definite.

This is because for all $\boldsymbol{x}$, we have

$$\boldsymbol{x}'\boldsymbol{P}'\boldsymbol{Px} = (\boldsymbol{Px})'(\boldsymbol{Px}) = \|\boldsymbol{Px}\|^2 \geq 0$$

Or we can use (6) and set $\boldsymbol{A} = \boldsymbol{I}$.

If $r(\boldsymbol{P}) = m$, then $\boldsymbol{P}'\boldsymbol{P}$ is positive definite. If $r(\boldsymbol{P}) = n$, then $\boldsymbol{PP}'$ is positive definite.
8. If $\boldsymbol{D}$ is diagonal and $\boldsymbol{P}$ is non-singular, then

$$\boldsymbol{P}'\boldsymbol{DP}: NND \Longleftrightarrow d_i \geq 0$$

9. If $\boldsymbol{A}$ is symmetric and idempotent, then $\boldsymbol{A}$ is NND.

$$\boldsymbol{A} = \boldsymbol{A}^2 = \boldsymbol{A}'\boldsymbol{A}: NND$$

The only positive definite symmetric idempotent matrix is $\boldsymbol{I}$. Any other projection matrix is positive semi-definite, i.e. singular.

## Decomposition of symmetric matrices

### Theorem 1

If $\boldsymbol{A}$ is a symmetric matrix, then there exists a non-singular matrix $\boldsymbol{Q}$ such that $\boldsymbol{Q}'\boldsymbol{AQ}$ is diagonal.

For example,

$$\boldsymbol{A} = \begin{pmatrix} 2 & 1 \\ 1 & 3 \end{pmatrix}, \quad \boldsymbol{Q} = \begin{pmatrix} a & b \\ c & d \end{pmatrix}$$

The $\boldsymbol{Q}$ matrix is not unique and there’s always a triangular $\boldsymbol{Q}$.

\begin{aligned} \boldsymbol{Q}'\boldsymbol{AQ} &= \begin{pmatrix} a & 0 \\ b & d \end{pmatrix} \begin{pmatrix} 2 & 1 \\ 1 & 3 \end{pmatrix} \begin{pmatrix} a & b \\ 0 & d \end{pmatrix} \\ &= \begin{pmatrix} 2a^2 & 2ab + ad \\ 2ab + ad & \cdots \end{pmatrix} \end{aligned}

For this to be diagonal, we need $2ab+ad=0$, so

$$\begin{cases} a=0 \\ d = -2b \end{cases} \quad \text{or} \quad \boldsymbol{Q}: \text{ singular}$$

We want the $\boldsymbol{Q}$ matrix to be singular, so the first case yields

$$\boldsymbol{Q} = \begin{pmatrix} a & b \\ 0 & -2b \end{pmatrix} \xlongequal{e.g.} \begin{pmatrix} 1 & 1 \\ 0 & -2 \end{pmatrix}$$

We’re not proving this theorem here. An important corollary of this is if $\boldsymbol{A}$ is symmetric, then there exists a non-singular matrix $\boldsymbol{P}$ and a diagonal matrix $\boldsymbol{D}$ such that $\boldsymbol{A} = \boldsymbol{P}'\boldsymbol{DP}$. To show this, set $\boldsymbol{P} = \boldsymbol{Q}^{-1}$ and use the theorem:

$$\begin{gathered} \boldsymbol{Q}'\boldsymbol{AQ} = \boldsymbol{D} \\ \boldsymbol{Q}'\boldsymbol{A} = \boldsymbol{DQ}^{-1} \\ \boldsymbol{A} = (\boldsymbol{Q}')^{-1}\boldsymbol{DQ}^{-1} \\ \boldsymbol{A} = (\boldsymbol{Q}^{-1})'\boldsymbol{DQ}^{-1} \end{gathered}$$

### Theorem 2

$\boldsymbol{A}$ is a symmetric non-negative definite matrix and $r(\boldsymbol{A}) = r$ $\Longleftrightarrow$ there exists $\boldsymbol{B}: r \times n$ and $r(\boldsymbol{B}) = r$ such that $\boldsymbol{A} = \boldsymbol{B}'\boldsymbol{B}$.

To prove ($\Leftarrow$), see that $\boldsymbol{B}'\boldsymbol{B}$ is non-negative definite when $\boldsymbol{B}$ is rank $r$.

To see ($\Rightarrow$), by the corollary in Theorem 1 we have $\boldsymbol{A} = \boldsymbol{PDP}'$ for some non-singular and diagonal $\boldsymbol{D}$. Since $\boldsymbol{PDP}'$ is non-negative definite, $d_i \geq 0$ for all $i$. Since $r(\boldsymbol{A}) = r$, exactly $r$ of the diagonals are positive, and $(n-r)$ of the diagonals are zeros. We can rearrange the $d_i$’s such that

$$d_1 \geq d_2 \geq \cdots \geq d_r > d_{r+1} = \cdots = d_n = 0$$

and we rearrange the columns of $\boldsymbol{P}$ accordingly. Then we take

$$\boldsymbol{B}'_{n \times r} = [\text{first } r \text{ columns of } \boldsymbol{P}] \begin{pmatrix} \sqrt{d_1} & 0 & \cdots & 0 \\ 0 & \sqrt{d_2} & \cdots & 0 \\ \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & \cdots & \sqrt{d_r} \end{pmatrix}$$

from which we can see

$$\boldsymbol{B} = \begin{pmatrix} \sqrt{d_1} & 0 & \cdots & 0 \\ 0 & \sqrt{d_2} & \cdots & 0 \\ \vdots & \vdots & \ddots & \vdots \\ 0 & 0 & \cdots & \sqrt{d_r} \end{pmatrix}_{r \times r} \begin{pmatrix} \boldsymbol{p}_1' \\ \vdots \\ \boldsymbol{p}_r' \end{pmatrix}_{r \times n}$$

Now $\boldsymbol{B}'\boldsymbol{B}$ would be

\begin{aligned} \boldsymbol{B}'\boldsymbol{B} &= [\boldsymbol{p}_1, \cdots, \boldsymbol{p}_r] \begin{pmatrix} \sqrt{d_1} & & \\ & \ddots & \\ & & \sqrt{d_r} \end{pmatrix} \begin{pmatrix} \sqrt{d_1} & & \\ & \ddots & \\ & & \sqrt{d_r} \end{pmatrix} \begin{pmatrix} \boldsymbol{p}_1' \\ \vdots \\ \boldsymbol{p}_r' \end{pmatrix} \\ &= [\boldsymbol{p}_1, \cdots, \boldsymbol{p}_n] \left[\begin{array}{ccc|c} d_1 & & & \\ & \ddots & & \huge0 \\ & & d_r & \\ \hline \\ & \huge0 & & \huge0 \end{array}\right] \begin{pmatrix} \boldsymbol{p}_1' \\ \vdots \\ \boldsymbol{p}_n' \end{pmatrix} \\ &= \boldsymbol{PDP}' = \boldsymbol{A} \end{aligned}

This decomposition is important in that it’s related to the eigendecomposition that we’ll see later.

#### Corollary

$$\boldsymbol{A}: \text{symmetric and NND} \Longleftrightarrow \exists \boldsymbol{B}: n \times n \text{ s.t. } \boldsymbol{A} = \boldsymbol{B}'\boldsymbol{B}$$

Another version of this corollary is

$$\boldsymbol{A}: \text{symmetric, NND, and } r(\boldsymbol{A}) = r \Longleftrightarrow \exists \boldsymbol{B}: m \times n (m \geq r) \text{ s.t. } \boldsymbol{A} = \boldsymbol{B}'\boldsymbol{B}$$

Although there seems no restrictions on the rank of $\boldsymbol{B}$, see that if we have the left side, then we cannot hava $\boldsymbol{A} = \boldsymbol{B}'\boldsymbol{B}$ where $\boldsymbol{B}$ is $m \times n$ and $m < r$. The rank of $\boldsymbol{B}$ must be $r$.

#### Properties

1. If $\boldsymbol{A}$ is symmetric and NND, then

$$\boldsymbol{Ax} = \boldsymbol{0} \Longleftrightarrow \boldsymbol{x}'\boldsymbol{Ax} = 0$$

Proving ($\Rightarrow$) is trivial. For the other direction, see that $\boldsymbol{A} = \boldsymbol{P}'\boldsymbol{P}$ for some $\boldsymbol{P}$.

$$\boldsymbol{x}'\boldsymbol{Ax} = \boldsymbol{x}'\boldsymbol{P}'\boldsymbol{Px} = \|\boldsymbol{Px}\|^2 = 0$$

This means $\boldsymbol{Px} = \boldsymbol{0}$, thus $\boldsymbol{Ax} = \boldsymbol{P}'\boldsymbol{Px} = \boldsymbol{0}$.

2. $\boldsymbol{A}$ is symmetric and positive definite $\Longleftrightarrow$ there exists a non-singular matrix $\boldsymbol{P}$ such that $\boldsymbol{A} = \boldsymbol{P}'\boldsymbol{P}$.

3. The inverse of a positive definite matrix $\boldsymbol{A}$ is positive definite:

$$\boldsymbol{A}^{-1} = \boldsymbol{P}^{-1}(\boldsymbol{P}')^{-1} = \boldsymbol{P}^{-1}(\boldsymbol{P}^{-1})'$$

What if $\boldsymbol{A} = \boldsymbol{P}'\boldsymbol{DP}$ is a positive semi-definite matrix? Similarly $\boldsymbol{P}$ is non-singular and $d_i \geq 0$ with exactly $r$ elements being positive and the others being zero. We want to find the generalized inverse of $\boldsymbol{A}$.

We rearrange the diagonals of $\boldsymbol{D}$ such that

$$\boldsymbol{D} = diag(d_1, \cdots, d_r, 0, \cdots, 0)$$

One way to find a generalized inverse is to define

$$\begin{gathered} \boldsymbol{D}^* = diag\left(\frac{1}{d_1}, \cdots, \frac{1}{d_r}, 0, \cdots, 0 \right) \\ \boldsymbol{G} \triangleq \boldsymbol{P}^{-1}\boldsymbol{D}^* (\boldsymbol{P}')^{-1} \end{gathered}$$

Then we have

$$\boldsymbol{AGA} = \boldsymbol{P}'\boldsymbol{DPP}^{-1}\boldsymbol{D}^* (\boldsymbol{P}')^{-1}\boldsymbol{P}'\boldsymbol{DP} = \boldsymbol{P}'\boldsymbol{DP} = \boldsymbol{A}$$

So $\boldsymbol{G}$ is also NND. Note that this means we can always find a positive semi-definite generalized inverse for $\boldsymbol{A}$. This doesn’t mean all $\boldsymbol{G}$’s are PSD!

4. If $\boldsymbol{A}$ is symmetric and non-negative definite, and $tr(\boldsymbol{A}) = 0$, then $\boldsymbol{A} = \boldsymbol{0}$.

From the fact that $\boldsymbol{A}$ is NND, we can rewrite it as $\boldsymbol{A} = \boldsymbol{BB}'$ for some $\boldsymbol{B}$. We then have

$$tr(\boldsymbol{BB}') = \sum_{i, j} b_{ij}^2 = 0 \Rightarrow \boldsymbol{B} = \boldsymbol{0} \Rightarrow \boldsymbol{A} = \boldsymbol{0}$$

5. If $\boldsymbol{A}$ is positive definite and $\boldsymbol{B}$ is non-negative definite and symmetric3, $tr(\boldsymbol{AB}) > 0$.

If the rank of $\boldsymbol{B}$ is $r$, we can rewrite $\boldsymbol{B} = \boldsymbol{Q}'\boldsymbol{Q}$ where $\boldsymbol{Q}$ is $r \times n$. Then,

$$tr(\boldsymbol{AB}) = tr(\boldsymbol{AQ}'\boldsymbol{Q}) = tr(\boldsymbol{QAQ}')$$

$\boldsymbol{QAQ}'$ is a positive definite matrix, and its trace is strictly positive.

1. $\boldsymbol{A}$ is an $n \times n$ symmetric non-negative definite matrix4, and we partition it into

$$\boldsymbol{A} = \begin{pmatrix} \boldsymbol{T}_{r \times r} & \boldsymbol{U}_{r \times s} \\ \boldsymbol{V}_{s \times r} & \boldsymbol{W}_{s \times s} \end{pmatrix}$$

where $\boldsymbol{T}$ and $\boldsymbol{W}$ are square. Then,

$$\begin{gathered} \mathcal{C}(\boldsymbol{U}) \subset \mathcal{C}(\boldsymbol{T}), \quad \mathcal{R}(\boldsymbol{V}) \subset \mathcal{R}(\boldsymbol{T}) \\ \mathcal{C}(\boldsymbol{V}) \subset \mathcal{C}(\boldsymbol{W}), \quad \mathcal{R}(\boldsymbol{U}) \subset \mathcal{R}(\boldsymbol{W}) \end{gathered}$$

To prove these, again we can rewrite $\boldsymbol{A} = \boldsymbol{B}'\boldsymbol{B}$ where $\boldsymbol{B}: n \times n$. Consider the partition $\boldsymbol{B} = [\boldsymbol{E}_{n \times r} \quad \boldsymbol{F}_{n \times s}]$. Matrix $\boldsymbol{A}$ is

$$\boldsymbol{A} = \begin{pmatrix} \boldsymbol{E}' \\ \boldsymbol{F}' \end{pmatrix} [\boldsymbol{E} \quad \boldsymbol{F}] = \begin{pmatrix} \boldsymbol{E}'\boldsymbol{E} & \boldsymbol{E}'\boldsymbol{F} \\ \boldsymbol{F}'\boldsymbol{E} & \boldsymbol{F}'\boldsymbol{F} \end{pmatrix} = \begin{pmatrix} \boldsymbol{T} & \boldsymbol{U} \\ \boldsymbol{V} & \boldsymbol{W} \end{pmatrix}$$

This is pretty much the end of the proof. For example, $\mathcal{C}(\boldsymbol{U}) = \mathcal{C}(\boldsymbol{E}'\boldsymbol{F}) \subset \mathcal{C}(\boldsymbol{E}')$, and $\mathcal{C}(\boldsymbol{T}) = \mathcal{C}(\boldsymbol{E}'\boldsymbol{E}) = \mathcal{C}(\boldsymbol{E}')$.

1. Some properties about the determinant:

• If $\boldsymbol{A}$ is positive definite, then $|\boldsymbol{A}| > 0$.

One way to prove this is to write $\boldsymbol{A}$ as $\boldsymbol{BB}'$ where $\boldsymbol{B}$ is non=singular. $|\boldsymbol{A}| = |\boldsymbol{B}||\boldsymbol{B}'| = |\boldsymbol{B}|^2 > 0$.

• If $\boldsymbol{A}$ is positive semi-definite, then $|\boldsymbol{A}| = 0$.

1. Just like when we first learned about solving $x-1=0$, where $x$ denotes some unknown number. ↩︎

2. These conclusions cannot be generalized to $\boldsymbol{A} - \boldsymbol{B}$ or $\boldsymbol{AB}$. ↩︎

3. $\boldsymbol{AB}$ is not necessarily non-negative definite. ↩︎

4. In the statistical context, $\boldsymbol{A}$ is a variance-covariance matrix, $\boldsymbol{T}$ is the covariance matrix of part of the variables, $\boldsymbol{W}$ is the covariance matrix of the other part of the variables, and $\boldsymbol{U}$ and $\boldsymbol{V}$ are the cross-covariance matrices. ↩︎

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