Proofs Chapter 7: Determinant Derivatives

Proof

We recall the cofactor expansion of a determinant (A.13). The determinant can be expanded along any row $i$ as the sum of products of row elements and their corresponding cofactors.

\begin{equation}|\boldsymbol{X}| = \sum_{j=0}^{N-1} X_{ij} \tilde{X}_{ij}\label{eq:7-1-1}\end{equation}

Here $\tilde{X}_{ij}$ is the $(i, j)$ cofactor of $\boldsymbol{X}$.

We recall the definition of a cofactor. $\tilde{X}_{ij}$ is the determinant of the $(N-1) \times (N-1)$ submatrix obtained by removing the $i$-th row and $j$-th column from $\boldsymbol{X}$ (the minor), multiplied by the sign $(-1)^{i+j}$.

\begin{equation}\tilde{X}_{ij} = (-1)^{i+j} M_{ij}\label{eq:7-1-2}\end{equation}

Here $M_{ij}$ is the $(i, j)$ minor.

A key property is that the cofactor $\tilde{X}_{ij}$ does not involve $X_{ij}$ itself. This is because the minor $M_{ij}$ is computed by excluding the $i$-th row and $j$-th column.

We differentiate the determinant $|\boldsymbol{X}|$ with respect to the component $X_{ij}$. Using the cofactor expansion \eqref{eq:7-1-1}, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial X_{ij}} |\boldsymbol{X}| = \dfrac{\partial}{\partial X_{ij}} \sum_{k=0}^{N-1} X_{ik} \tilde{X}_{ik}\label{eq:7-1-3}\end{equation}

We decompose the derivative of the sum into the sum of derivatives of each term.

\begin{equation}\dfrac{\partial}{\partial X_{ij}} \sum_{k=0}^{N-1} X_{ik} \tilde{X}_{ik} = \sum_{k=0}^{N-1} \dfrac{\partial}{\partial X_{ij}} (X_{ik} \tilde{X}_{ik})\label{eq:7-1-4}\end{equation}

We compute the derivative of each term. Since the cofactor $\tilde{X}_{ik}$ does not contain $X_{ij}$, it can be treated as a constant. Therefore the following holds.

\begin{equation}\dfrac{\partial}{\partial X_{ij}} (X_{ik} \tilde{X}_{ik}) = \tilde{X}_{ik} \dfrac{\partial X_{ik}}{\partial X_{ij}}\label{eq:7-1-5}\end{equation}

We compute the partial derivative of the component. $X_{ik}$ equals $X_{ij}$ only when $k = j$.

\begin{equation}\dfrac{\partial X_{ik}}{\partial X_{ij}} = \delta_{kj}\label{eq:7-1-6}\end{equation}

Here $\delta_{kj}$ is the Kronecker delta, which equals 1 when $k = j$ and 0 otherwise.

Combining \eqref{eq:7-1-5} and \eqref{eq:7-1-6}, we obtain the following.

\begin{equation}\sum_{k=0}^{N-1} \tilde{X}_{ik} \delta_{kj} = \tilde{X}_{ij}\label{eq:7-1-7}\end{equation}

Since $\delta_{kj} = 1$ only when $k = j$, only the $k = j$ term survives in the sum.

Thus, the component-wise derivative of the determinant equals the cofactor.

\begin{equation}\dfrac{\partial}{\partial X_{ij}} |\boldsymbol{X}| = \tilde{X}_{ij}\label{eq:7-1-8}\end{equation}

We consider the matrix derivative in the denominator layout. In the denominator layout, the $(i, j)$ component of the result corresponds to $\dfrac{\partial}{\partial X_{ij}}$.

\begin{equation}\left( \dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}| \right)_{ij} = \dfrac{\partial}{\partial X_{ij}} |\boldsymbol{X}| = \tilde{X}_{ij}\label{eq:7-1-9}\end{equation}

We write this in matrix form. We define the adjugate matrix $\text{adj}(\boldsymbol{X})$. The $(i, j)$ component of $\text{adj}(\boldsymbol{X})$ is $\tilde{X}_{ji}$ (note the swapped indices).

Writing the result of \eqref{eq:7-1-9} as a matrix, since the $(i, j)$ component is $\tilde{X}_{ij}$, this is the transpose of $\text{adj}(\boldsymbol{X})$, namely $\text{adj}(\boldsymbol{X})^\top$.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}| = \text{adj}(\boldsymbol{X})^\top\label{eq:7-1-10}\end{equation}

We rewrite using the relationship between the inverse matrix and the adjugate matrix. For an invertible matrix $\boldsymbol{X}$, the following relation holds.

\begin{equation}\boldsymbol{X}^{-1} = \dfrac{1}{|\boldsymbol{X}|} \text{adj}(\boldsymbol{X})\label{eq:7-1-11}\end{equation}

Solving \eqref{eq:7-1-11} for $\text{adj}(\boldsymbol{X})$, we obtain the following.

\begin{equation}\text{adj}(\boldsymbol{X}) = |\boldsymbol{X}| \boldsymbol{X}^{-1}\label{eq:7-1-12}\end{equation}

Taking the transpose of both sides.

\begin{equation}\text{adj}(\boldsymbol{X})^\top = (|\boldsymbol{X}| \boldsymbol{X}^{-1})^\top\label{eq:7-1-13}\end{equation}

The scalar $|\boldsymbol{X}|$ is unchanged by transposition. Also, the transpose of a product is $(AB)^\top = B^\top A^\top$, but since this is a product with a scalar, the order does not matter. Therefore the following holds.

\begin{equation}\text{adj}(\boldsymbol{X})^\top = |\boldsymbol{X}| (\boldsymbol{X}^{-1})^\top = |\boldsymbol{X}| \boldsymbol{X}^{-\top}\label{eq:7-1-14}\end{equation}

Combining \eqref{eq:7-1-10} and \eqref{eq:7-1-14}, we obtain the final result.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}| = |\boldsymbol{X}| \boldsymbol{X}^{-\top}\label{eq:7-1-15}\end{equation}

Proof

The log-determinant is a composite function $\log(|\boldsymbol{X}|)$. The outer function is $\log$ and the inner function is $|\boldsymbol{X}|$.

We apply the chain rule for composite functions. The matrix derivative of a scalar function $f(g(\boldsymbol{X}))$ is as follows.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} f(g(\boldsymbol{X})) = f'(g(\boldsymbol{X})) \dfrac{\partial g}{\partial \boldsymbol{X}}\label{eq:7-2-1}\end{equation}

In this problem, $f(u) = \log(u)$ and $g(\boldsymbol{X}) = |\boldsymbol{X}|$.

Computing the derivative of the outer function $f(u) = \log(u)$, we obtain the following.

\begin{equation}f'(u) = \dfrac{1}{u}\label{eq:7-2-2}\end{equation}

Substituting $u = g(\boldsymbol{X}) = |\boldsymbol{X}|$, we obtain the following.

\begin{equation}f'(g(\boldsymbol{X})) = f'(|\boldsymbol{X}|) = \dfrac{1}{|\boldsymbol{X}|}\label{eq:7-2-3}\end{equation}

The derivative of the inner function is given by 7.1 as follows.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}| = |\boldsymbol{X}| \boldsymbol{X}^{-\top}\label{eq:7-2-4}\end{equation}

Substituting \eqref{eq:7-2-3} and \eqref{eq:7-2-4} into the chain rule \eqref{eq:7-2-1}, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} \log|\boldsymbol{X}| = \dfrac{1}{|\boldsymbol{X}|} \cdot |\boldsymbol{X}| \boldsymbol{X}^{-\top}\label{eq:7-2-5}\end{equation}

$\dfrac{1}{|\boldsymbol{X}|}$ and $|\boldsymbol{X}|$ cancel, yielding the final result.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} \log|\boldsymbol{X}| = \boldsymbol{X}^{-\top}\label{eq:7-2-6}\end{equation}

Proof

We recall the multiplicative property of determinants (1.14). For any square matrices $\boldsymbol{A}$, $\boldsymbol{B}$, the following holds.

\begin{equation}|\boldsymbol{A}\boldsymbol{B}| = |\boldsymbol{A}||\boldsymbol{B}|\label{eq:7-3-1}\end{equation}

Applying this property repeatedly, the determinant of $\boldsymbol{X}^n = \underbrace{\boldsymbol{X} \cdot \boldsymbol{X} \cdots \boldsymbol{X}}_{n \text{ times}}$ becomes the following.

\begin{equation}|\boldsymbol{X}^n| = |\underbrace{\boldsymbol{X} \cdot \boldsymbol{X} \cdots \boldsymbol{X}}_{n \text{ times}}| = \underbrace{|\boldsymbol{X}| \cdot |\boldsymbol{X}| \cdots |\boldsymbol{X}|}_{n \text{ times}} = |\boldsymbol{X}|^n\label{eq:7-3-2}\end{equation}

Therefore, the problem of differentiating $|\boldsymbol{X}^n|$ with respect to $\boldsymbol{X}$ reduces to differentiating $|\boldsymbol{X}|^n$ with respect to $\boldsymbol{X}$.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}^n| = \dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}|^n\label{eq:7-3-3}\end{equation}

$|\boldsymbol{X}|^n$ is the $n$-th power of $|\boldsymbol{X}|$, which is a composite function. Let the outer function be $f(u) = u^n$ and the inner function be $g(\boldsymbol{X}) = |\boldsymbol{X}|$.

Computing the derivative of the outer function $f(u) = u^n$, we obtain the following.

\begin{equation}f'(u) = n u^{n-1}\label{eq:7-3-4}\end{equation}

Substituting $u = |\boldsymbol{X}|$, we obtain the following.

\begin{equation}f'(|\boldsymbol{X}|) = n |\boldsymbol{X}|^{n-1}\label{eq:7-3-5}\end{equation}

The derivative of the inner function is given by 7.1 as follows.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}| = |\boldsymbol{X}| \boldsymbol{X}^{-\top}\label{eq:7-3-6}\end{equation}

Applying the chain rule, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}|^n = n |\boldsymbol{X}|^{n-1} \cdot |\boldsymbol{X}| \boldsymbol{X}^{-\top}\label{eq:7-3-7}\end{equation}

Using $|\boldsymbol{X}|^{n-1} \cdot |\boldsymbol{X}| = |\boldsymbol{X}|^n$ and simplifying, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}|^n = n |\boldsymbol{X}|^n \boldsymbol{X}^{-\top}\label{eq:7-3-8}\end{equation}

Since $|\boldsymbol{X}|^n = |\boldsymbol{X}^n|$ by \eqref{eq:7-3-2}, we obtain the final result.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}^n| = n |\boldsymbol{X}^n| \boldsymbol{X}^{-\top}\label{eq:7-3-9}\end{equation}

Proof

$\det(\boldsymbol{Y})$ is a function of all components $Y_{ij}$ of $\boldsymbol{Y}$. Since each component $Y_{ij}$ is a function of the scalar $x$, $\det(\boldsymbol{Y})$ is a composite function of $x$.

We apply the multivariable chain rule. Differentiating $\det(\boldsymbol{Y})$ with respect to $x$ yields the sum of contributions through all intermediate variables $Y_{ij}$.

\begin{equation}\dfrac{\partial \det(\boldsymbol{Y})}{\partial x} = \sum_{i=0}^{N-1} \sum_{j=0}^{N-1} \dfrac{\partial \det(\boldsymbol{Y})}{\partial Y_{ij}} \dfrac{\partial Y_{ij}}{\partial x}\label{eq:7-4-1}\end{equation}

From the proof of 7.1, the component-wise derivative of the determinant equals the cofactor.

\begin{equation}\dfrac{\partial \det(\boldsymbol{Y})}{\partial Y_{ij}} = \tilde{Y}_{ij}\label{eq:7-4-2}\end{equation}

Here $\tilde{Y}_{ij}$ is the $(i, j)$ cofactor of $\boldsymbol{Y}$.

Substituting \eqref{eq:7-4-2} into \eqref{eq:7-4-1}, we obtain the following.

\begin{equation}\dfrac{\partial \det(\boldsymbol{Y})}{\partial x} = \sum_{i,j} \tilde{Y}_{ij} \dfrac{\partial Y_{ij}}{\partial x}\label{eq:7-4-3}\end{equation}

We use the relationship between cofactors and the inverse matrix. As shown in the proof of 7.1, the following relation holds.

\begin{equation}\text{adj}(\boldsymbol{Y}) = \det(\boldsymbol{Y}) \boldsymbol{Y}^{-1}\label{eq:7-4-4}\end{equation}

Here the $(i, j)$ component of $\text{adj}(\boldsymbol{Y})$ is $\tilde{Y}_{ji}$.

Therefore $\tilde{Y}_{ij}$ is the $(j, i)$ component of $\text{adj}(\boldsymbol{Y})$, so it can be written as follows.

\begin{equation}\tilde{Y}_{ij} = (\text{adj}(\boldsymbol{Y}))_{ji} = \det(\boldsymbol{Y}) (Y^{-1})_{ji}\label{eq:7-4-5}\end{equation}

Substituting \eqref{eq:7-4-5} into \eqref{eq:7-4-3}, we obtain the following.

\begin{equation}\dfrac{\partial \det(\boldsymbol{Y})}{\partial x} = \sum_{i,j} \det(\boldsymbol{Y}) (Y^{-1})_{ji} \dfrac{\partial Y_{ij}}{\partial x}\label{eq:7-4-6}\end{equation}

Since $\det(\boldsymbol{Y})$ does not depend on $i, j$, we factor it out of the sum.

\begin{equation}\dfrac{\partial \det(\boldsymbol{Y})}{\partial x} = \det(\boldsymbol{Y}) \sum_{i,j} (Y^{-1})_{ji} \dfrac{\partial Y_{ij}}{\partial x}\label{eq:7-4-7}\end{equation}

Interchanging the order of summation and summing over $i$ first, we obtain the following.

\begin{equation}\sum_{i,j} (Y^{-1})_{ji} \dfrac{\partial Y_{ij}}{\partial x} = \sum_{j=0}^{N-1} \sum_{i=0}^{N-1} (Y^{-1})_{ji} \dfrac{\partial Y_{ij}}{\partial x}\label{eq:7-4-8}\end{equation}

The inner sum $\displaystyle\sum_{i} (Y^{-1})_{ji} \dfrac{\partial Y_{ij}}{\partial x}$ corresponds to the definition of matrix multiplication. This is the $(j, j)$ component of $\boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial x}$.

\begin{equation}\sum_{i=0}^{N-1} (Y^{-1})_{ji} \left(\dfrac{\partial \boldsymbol{Y}}{\partial x}\right)_{ij} = \left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial x} \right)_{jj}\label{eq:7-4-9}\end{equation}

The outer sum $\sum_{j}$ is the sum of diagonal elements, i.e., the trace.

\begin{equation}\sum_{j=0}^{N-1} \left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial x} \right)_{jj} = \text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial x} \right)\label{eq:7-4-10}\end{equation}

Combining \eqref{eq:7-4-7} and \eqref{eq:7-4-10}, we obtain the final result (Jacobi's formula).

\begin{equation}\dfrac{\partial \det(\boldsymbol{Y})}{\partial x} = \det(\boldsymbol{Y}) \text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial x} \right)\label{eq:7-4-11}\end{equation}

Proof

From the proof of 7.1, the component-wise derivative of the determinant equals the cofactor.

\begin{equation}\dfrac{\partial \det(\boldsymbol{X})}{\partial X_{ik}} = \tilde{X}_{ik}\label{eq:7-5-1}\end{equation}

Using this result, we rewrite the sum on the left-hand side in terms of cofactors.

\begin{equation}\sum_{k=0}^{N-1} \dfrac{\partial \det(\boldsymbol{X})}{\partial X_{ik}} X_{jk} = \sum_{k=0}^{N-1} \tilde{X}_{ik} X_{jk}\label{eq:7-5-2}\end{equation}

We consider the cases $i = j$ and $i \neq j$ separately.

Case $i = j$. The sum becomes the following.

\begin{equation}\sum_{k=0}^{N-1} \tilde{X}_{ik} X_{ik}\label{eq:7-5-3}\end{equation}

This is precisely the cofactor expansion of the determinant. The expansion of the determinant along the $i$-th row is as follows.

\begin{equation}\det(\boldsymbol{X}) = \sum_{k=0}^{N-1} X_{ik} \tilde{X}_{ik}\label{eq:7-5-4}\end{equation}

Therefore, when $i = j$, the following holds.

\begin{equation}\sum_{k=0}^{N-1} \tilde{X}_{ik} X_{ik} = \det(\boldsymbol{X})\label{eq:7-5-5}\end{equation}

Case $i \neq j$. The sum becomes the following.

\begin{equation}\sum_{k=0}^{N-1} \tilde{X}_{ik} X_{jk}\label{eq:7-5-6}\end{equation}

We consider the meaning of this sum. $\tilde{X}_{ik}$ is the determinant of the submatrix obtained by removing the $i$-th row and $k$-th column from $\boldsymbol{X}$, multiplied by a sign. Therefore $\tilde{X}_{ik}$ does not contain any elements from the $i$-th row of $\boldsymbol{X}$.

The sum in \eqref{eq:7-5-6} equals the cofactor expansion along the $i$-th row of the matrix $\boldsymbol{X}'$ obtained by replacing the $i$-th row of $\boldsymbol{X}$ with the $j$-th row.

\begin{equation}\sum_{k=0}^{N-1} \tilde{X}_{ik} X_{jk} = \det(\boldsymbol{X}')\label{eq:7-5-7}\end{equation}

Here $\boldsymbol{X}'$ is the matrix obtained by replacing the $i$-th row of $\boldsymbol{X}$ with the $j$-th row of $\boldsymbol{X}$ (note that the cofactors $\tilde{X}_{ik}$ do not depend on the $i$-th row).

In $\boldsymbol{X}'$, the $i$-th row and the $j$-th row are the same vector. That is, two rows are identical.

As a property of determinants, the determinant of a matrix with two identical rows (or columns) is 0.

\begin{equation}\det(\boldsymbol{X}') = 0\label{eq:7-5-8}\end{equation}

Therefore, when $i \neq j$, the following holds.

\begin{equation}\sum_{k=0}^{N-1} \tilde{X}_{ik} X_{jk} = 0\label{eq:7-5-9}\end{equation}

Expressing \eqref{eq:7-5-5} and \eqref{eq:7-5-9} uniformly using the Kronecker delta, we obtain the final result.

\begin{equation}\sum_{k=0}^{N-1} \dfrac{\partial \det(\boldsymbol{X})}{\partial X_{ik}} X_{jk} = \delta_{ij} \det(\boldsymbol{X})\label{eq:7-5-10}\end{equation}

Proof

We restate Jacobi's formula from 7.4.

\begin{equation}\dfrac{\partial \det(\boldsymbol{Y})}{\partial x} = \det(\boldsymbol{Y}) \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}')\label{eq:7-6-1}\end{equation}

We differentiate both sides once more with respect to $x$. The left-hand side becomes $\dfrac{\partial^2 \det(\boldsymbol{Y})}{\partial x^2}$. The right-hand side is a product of two factors, so we apply the product rule (1.25).

\begin{equation}\dfrac{\partial^2 \det(\boldsymbol{Y})}{\partial x^2} = \dfrac{\partial}{\partial x} \left[ \det(\boldsymbol{Y}) \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}') \right]\label{eq:7-6-2}\end{equation}

Applying the product rule (1.25) $(fg)' = f'g + fg'$ with $f = \det(\boldsymbol{Y})$ and $g = \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}')$, we obtain the following.

\begin{equation}\dfrac{\partial^2 \det(\boldsymbol{Y})}{\partial x^2} = \dfrac{\partial \det(\boldsymbol{Y})}{\partial x} \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}') + \det(\boldsymbol{Y}) \dfrac{\partial}{\partial x} \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}')\label{eq:7-6-3}\end{equation}

We compute the first term. Substituting Jacobi's formula \eqref{eq:7-6-1} for $\dfrac{\partial \det(\boldsymbol{Y})}{\partial x}$, we obtain the following.

\begin{equation}\dfrac{\partial \det(\boldsymbol{Y})}{\partial x} \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}') = \det(\boldsymbol{Y}) \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}') \cdot \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}')\label{eq:7-6-4}\end{equation}

Simplifying the first term, we obtain the following.

\begin{equation}\det(\boldsymbol{Y}) \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}')^2\label{eq:7-6-5}\end{equation}

We compute $\dfrac{\partial}{\partial x} \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}')$ in the second term. Since the trace and differentiation commute, we first differentiate inside the trace.

\begin{equation}\dfrac{\partial}{\partial x} \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}') = \text{tr}\left( \dfrac{\partial}{\partial x} (\boldsymbol{Y}^{-1} \boldsymbol{Y}') \right)\label{eq:7-6-6}\end{equation}

Applying the product rule (1.25) to the matrix product $\boldsymbol{Y}^{-1} \boldsymbol{Y}'$, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial x} (\boldsymbol{Y}^{-1} \boldsymbol{Y}') = \dfrac{\partial \boldsymbol{Y}^{-1}}{\partial x} \boldsymbol{Y}' + \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}'}{\partial x}\label{eq:7-6-7}\end{equation}

$\dfrac{\partial \boldsymbol{Y}'}{\partial x} = \boldsymbol{Y}''$.

We apply the scalar derivative formula for the inverse. Differentiating $\boldsymbol{Y}\boldsymbol{Y}^{-1} = \boldsymbol{I}$ with respect to $x$, the following holds.

\begin{equation}\boldsymbol{Y}' \boldsymbol{Y}^{-1} + \boldsymbol{Y} \dfrac{\partial \boldsymbol{Y}^{-1}}{\partial x} = \boldsymbol{O}\label{eq:7-6-8}\end{equation}

Solving \eqref{eq:7-6-8} for $\dfrac{\partial \boldsymbol{Y}^{-1}}{\partial x}$, we obtain the following.

\begin{equation}\dfrac{\partial \boldsymbol{Y}^{-1}}{\partial x} = -\boldsymbol{Y}^{-1} \boldsymbol{Y}' \boldsymbol{Y}^{-1}\label{eq:7-6-9}\end{equation}

Substituting \eqref{eq:7-6-9} into \eqref{eq:7-6-7}, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial x} (\boldsymbol{Y}^{-1} \boldsymbol{Y}') = -\boldsymbol{Y}^{-1} \boldsymbol{Y}' \boldsymbol{Y}^{-1} \boldsymbol{Y}' + \boldsymbol{Y}^{-1} \boldsymbol{Y}''\label{eq:7-6-10}\end{equation}

We simplify the first term. Since $\boldsymbol{Y}^{-1} \boldsymbol{Y}'$ appears twice, it can be written as follows.

\begin{equation}-\boldsymbol{Y}^{-1} \boldsymbol{Y}' \boldsymbol{Y}^{-1} \boldsymbol{Y}' = -(\boldsymbol{Y}^{-1} \boldsymbol{Y}')^2\label{eq:7-6-11}\end{equation}

Taking the trace of \eqref{eq:7-6-6}, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial x} \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}') = \text{tr}(-(\boldsymbol{Y}^{-1} \boldsymbol{Y}')^2 + \boldsymbol{Y}^{-1} \boldsymbol{Y}'')\label{eq:7-6-12}\end{equation}

Using the linearity of trace to decompose, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial x} \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}') = -\text{tr}((\boldsymbol{Y}^{-1} \boldsymbol{Y}')^2) + \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}'')\label{eq:7-6-13}\end{equation}

Substituting the results of \eqref{eq:7-6-5} and \eqref{eq:7-6-13} into \eqref{eq:7-6-3}, we obtain the following.

\begin{equation}\dfrac{\partial^2 \det(\boldsymbol{Y})}{\partial x^2} = \det(\boldsymbol{Y}) \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}')^2 + \det(\boldsymbol{Y}) \left[ -\text{tr}((\boldsymbol{Y}^{-1} \boldsymbol{Y}')^2) + \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}'') \right]\label{eq:7-6-14}\end{equation}

Factoring out $\det(\boldsymbol{Y})$, we obtain the final result.

\begin{equation}\dfrac{\partial^2 \det(\boldsymbol{Y})}{\partial x^2} = \det(\boldsymbol{Y}) \left[ \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}'') + \text{tr}(\boldsymbol{Y}^{-1} \boldsymbol{Y}')^2 - \text{tr}((\boldsymbol{Y}^{-1} \boldsymbol{Y}')^2) \right]\label{eq:7-6-15}\end{equation}

Proof

By the multiplicative property of determinants (1.14), the determinant of a product of square matrices equals the product of their determinants.

\begin{equation}|\boldsymbol{A}\boldsymbol{B}| = |\boldsymbol{A}||\boldsymbol{B}|\label{eq:7-7-1}\end{equation}

We apply this property to $\boldsymbol{A}\boldsymbol{X}\boldsymbol{B}$. Viewing it as $(\boldsymbol{A}\boldsymbol{X})\boldsymbol{B}$, we obtain the following.

\begin{equation}|(\boldsymbol{A}\boldsymbol{X})\boldsymbol{B}| = |\boldsymbol{A}\boldsymbol{X}||\boldsymbol{B}|\label{eq:7-7-2}\end{equation}

Applying the same property to $|\boldsymbol{A}\boldsymbol{X}|$, we obtain the following.

\begin{equation}|\boldsymbol{A}\boldsymbol{X}| = |\boldsymbol{A}||\boldsymbol{X}|\label{eq:7-7-3}\end{equation}

Combining \eqref{eq:7-7-2} and \eqref{eq:7-7-3}, we obtain the following.

\begin{equation}|\boldsymbol{A}\boldsymbol{X}\boldsymbol{B}| = |\boldsymbol{A}||\boldsymbol{X}||\boldsymbol{B}|\label{eq:7-7-4}\end{equation}

We differentiate $|\boldsymbol{A}\boldsymbol{X}\boldsymbol{B}|$ with respect to $\boldsymbol{X}$. Since $|\boldsymbol{A}|$ and $|\boldsymbol{B}|$ do not depend on $\boldsymbol{X}$, they can be factored out of the differentiation operator.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{A}\boldsymbol{X}\boldsymbol{B}| = \dfrac{\partial}{\partial \boldsymbol{X}} (|\boldsymbol{A}||\boldsymbol{X}||\boldsymbol{B}|) = |\boldsymbol{A}||\boldsymbol{B}| \dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}|\label{eq:7-7-5}\end{equation}

Substituting $\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}| = |\boldsymbol{X}| \boldsymbol{X}^{-\top}$ from 7.1, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{A}\boldsymbol{X}\boldsymbol{B}| = |\boldsymbol{A}||\boldsymbol{B}| \cdot |\boldsymbol{X}| \boldsymbol{X}^{-\top}\label{eq:7-7-6}\end{equation}

Using $|\boldsymbol{A}||\boldsymbol{B}||\boldsymbol{X}| = |\boldsymbol{A}\boldsymbol{X}\boldsymbol{B}|$ (from \eqref{eq:7-7-4}) and simplifying, we obtain the final result.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{A}\boldsymbol{X}\boldsymbol{B}| = |\boldsymbol{A}\boldsymbol{X}\boldsymbol{B}| \boldsymbol{X}^{-\top}\label{eq:7-7-7}\end{equation}

Proof

We decompose $|\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}|$ using the multiplicative property of determinants (1.14).

\begin{equation}|\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}| = |\boldsymbol{X}^\top||\boldsymbol{A}||\boldsymbol{X}|\label{eq:7-8-1-1}\end{equation}

By the determinant of the transpose (1.15), $|\boldsymbol{X}^\top| = |\boldsymbol{X}|$.

From \eqref{eq:7-8-1-1} and this property, we obtain the following.

\begin{equation}|\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}| = |\boldsymbol{X}||\boldsymbol{A}||\boldsymbol{X}| = |\boldsymbol{A}||\boldsymbol{X}|^2\label{eq:7-8-1-2}\end{equation}

We differentiate with respect to $\boldsymbol{X}$. Since $|\boldsymbol{A}|$ is a constant, it can be factored out of the differentiation operator.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}| = |\boldsymbol{A}| \dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}|^2\label{eq:7-8-1-3}\end{equation}

From 7.3 with $n = 2$, the following holds.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}|^2 = 2 |\boldsymbol{X}|^2 \boldsymbol{X}^{-\top}\label{eq:7-8-1-4}\end{equation}

Substituting \eqref{eq:7-8-1-4} into \eqref{eq:7-8-1-3}, we obtain the following.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}| = |\boldsymbol{A}| \cdot 2 |\boldsymbol{X}|^2 \boldsymbol{X}^{-\top}\label{eq:7-8-1-5}\end{equation}

Using $|\boldsymbol{A}||\boldsymbol{X}|^2 = |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}|$ (from \eqref{eq:7-8-1-2}) and simplifying, we obtain the final result.

\begin{equation}\dfrac{\partial}{\partial \boldsymbol{X}} |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}| = 2 |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}| \boldsymbol{X}^{-\top}\label{eq:7-8-1-6}\end{equation}

Proof

Let $\boldsymbol{Y} = \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}$. $\boldsymbol{Y}$ is an $N \times N$ matrix. Since $\boldsymbol{X}$ has full rank, $\boldsymbol{Y}$ is invertible (when $\boldsymbol{A}$ is positive definite).

Using Jacobi's formula from 7.4, differentiating $|\boldsymbol{Y}|$ with respect to component $X_{ij}$ gives the following.

\begin{equation}\dfrac{\partial |\boldsymbol{Y}|}{\partial X_{ij}} = |\boldsymbol{Y}| \text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} \right)\label{eq:7-8-2-1}\end{equation}

We differentiate $\boldsymbol{Y} = \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}$ with respect to $X_{ij}$. Since $\boldsymbol{X}$ appears in two places, $\boldsymbol{X}^\top$ and $\boldsymbol{X}$, we apply the product rule (1.25).

\begin{equation}\dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} = \dfrac{\partial (\boldsymbol{X}^\top)}{\partial X_{ij}} \boldsymbol{A} \boldsymbol{X} + \boldsymbol{X}^\top \boldsymbol{A} \dfrac{\partial \boldsymbol{X}}{\partial X_{ij}}\label{eq:7-8-2-2}\end{equation}

We compute $\dfrac{\partial \boldsymbol{X}}{\partial X_{ij}}$. Differentiating the component $X_{kl}$ of $\boldsymbol{X}$ with respect to $X_{ij}$ gives 1 only when $k = i$ and $l = j$, and 0 otherwise.

\begin{equation}\left( \dfrac{\partial \boldsymbol{X}}{\partial X_{ij}} \right)_{kl} = \delta_{ki} \delta_{lj}\label{eq:7-8-2-3}\end{equation}

We express this result as a matrix. Using the standard basis vectors $\boldsymbol{e}_i \in \mathbb{R}^M$ (with only the $i$-th component equal to 1) and $\boldsymbol{e}_j \in \mathbb{R}^N$ (with only the $j$-th component equal to 1), it can be written as follows.

\begin{equation}\dfrac{\partial \boldsymbol{X}}{\partial X_{ij}} = \boldsymbol{e}_i \boldsymbol{e}_j^\top\label{eq:7-8-2-4}\end{equation}

This is an $M \times N$ matrix with only the $(i, j)$ component equal to 1 and all others 0.

We compute $\dfrac{\partial (\boldsymbol{X}^\top)}{\partial X_{ij}}$. Taking the transpose swaps rows and columns, so we obtain the following.

\begin{equation}\dfrac{\partial (\boldsymbol{X}^\top)}{\partial X_{ij}} = \left( \dfrac{\partial \boldsymbol{X}}{\partial X_{ij}} \right)^\top = (\boldsymbol{e}_i \boldsymbol{e}_j^\top)^\top = \boldsymbol{e}_j \boldsymbol{e}_i^\top\label{eq:7-8-2-5}\end{equation}

Substituting \eqref{eq:7-8-2-4} and \eqref{eq:7-8-2-5} into \eqref{eq:7-8-2-2}, we obtain the following.

\begin{equation}\dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} = \boldsymbol{e}_j \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} + \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \boldsymbol{e}_j^\top\label{eq:7-8-2-6}\end{equation}

We use the symmetry of $\boldsymbol{A}$. Since $\boldsymbol{A} = \boldsymbol{A}^\top$, we have $\boldsymbol{A} \boldsymbol{e}_i = \boldsymbol{A}^\top \boldsymbol{e}_i$.

Note that $\boldsymbol{e}_i^\top \boldsymbol{A} = (\boldsymbol{A}^\top \boldsymbol{e}_i)^\top = (\boldsymbol{A} \boldsymbol{e}_i)^\top$. $\boldsymbol{A} \boldsymbol{e}_i$ is the $i$-th column vector of $\boldsymbol{A}$.

We analyze the first term of \eqref{eq:7-8-2-6}. $\boldsymbol{e}_j \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X}$ is an $N \times N$ matrix.

Substituting into \eqref{eq:7-8-2-1} and computing the trace, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} \right) = \text{tr}\left( \boldsymbol{Y}^{-1} \boldsymbol{e}_j \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \right) + \text{tr}\left( \boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \boldsymbol{e}_j^\top \right)\label{eq:7-8-2-7}\end{equation}

Applying the cyclic property of trace $\text{tr}(\boldsymbol{P}\boldsymbol{Q}\boldsymbol{R}) = \text{tr}(\boldsymbol{R}\boldsymbol{P}\boldsymbol{Q})$ to the first term, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \boldsymbol{e}_j \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \right) = \text{tr}\left( \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1} \boldsymbol{e}_j \right)\label{eq:7-8-2-8}\end{equation}

$\boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1} \boldsymbol{e}_j$ is a $1 \times 1$ scalar, so the trace equals the value itself.

\begin{equation}\text{tr}\left( \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1} \boldsymbol{e}_j \right) = \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1} \boldsymbol{e}_j = (\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1})_{ij}\label{eq:7-8-2-9}\end{equation}

Similarly, applying the cyclic property of trace to the second term, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \boldsymbol{e}_j^\top \right) = \text{tr}\left( \boldsymbol{e}_j^\top \boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \right)\label{eq:7-8-2-10}\end{equation}

This is also a $1 \times 1$ scalar, so the trace equals the value itself.

\begin{equation}\text{tr}\left( \boldsymbol{e}_j^\top \boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \right) = (\boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A})_{ji} = ((\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-\top}))_{ij}\label{eq:7-8-2-11}\end{equation}

We verify that $\boldsymbol{Y}$ is symmetric. Since $\boldsymbol{A} = \boldsymbol{A}^\top$, the following holds.

\begin{equation}\boldsymbol{Y}^\top = (\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X})^\top = \boldsymbol{X}^\top \boldsymbol{A}^\top (\boldsymbol{X}^\top)^\top = \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X} = \boldsymbol{Y}\label{eq:7-8-2-12}\end{equation}

Since $\boldsymbol{Y}$ is symmetric, $\boldsymbol{Y}^{-1}$ is also symmetric. Therefore $\boldsymbol{Y}^{-\top} = \boldsymbol{Y}^{-1}$.

Using the results of \eqref{eq:7-8-2-9} and \eqref{eq:7-8-2-11} and simplifying \eqref{eq:7-8-2-7}, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} \right) = (\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1})_{ij} + (\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1})_{ij} = 2(\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1})_{ij}\label{eq:7-8-2-13}\end{equation}

Substituting into \eqref{eq:7-8-2-1}, we obtain the derivative with respect to component $X_{ij}$.

\begin{equation}\dfrac{\partial |\boldsymbol{Y}|}{\partial X_{ij}} = |\boldsymbol{Y}| \cdot 2(\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1})_{ij} = 2|\boldsymbol{Y}| (\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1})_{ij}\label{eq:7-8-2-14}\end{equation}

In the denominator layout, the $(i, j)$ component of the result corresponds to $\dfrac{\partial}{\partial X_{ij}}$. Therefore, in matrix form, we obtain the following.

\begin{equation}\dfrac{\partial |\boldsymbol{Y}|}{\partial \boldsymbol{X}} = 2|\boldsymbol{Y}| \boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1}\label{eq:7-8-2-15}\end{equation}

Substituting $\boldsymbol{Y} = \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}$, we obtain the final result.

\begin{equation}\dfrac{\partial |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}|}{\partial \boldsymbol{X}} = 2 |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}| \boldsymbol{A} \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X})^{-1}\label{eq:7-8-2-16}\end{equation}

Proof

Let $\boldsymbol{Y} = \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}$. $\boldsymbol{Y}$ is an $N \times N$ matrix.

Using Jacobi's formula from 7.4, we obtain the following.

\begin{equation}\dfrac{\partial |\boldsymbol{Y}|}{\partial X_{ij}} = |\boldsymbol{Y}| \text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} \right)\label{eq:7-8-3-1}\end{equation}

As in 7.8.2, differentiating $\boldsymbol{Y}$ with respect to $X_{ij}$ gives the following.

\begin{equation}\dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} = \boldsymbol{e}_j \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} + \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \boldsymbol{e}_j^\top\label{eq:7-8-3-2}\end{equation}

Splitting the trace into two terms and computing, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} \right) = \text{tr}\left( \boldsymbol{Y}^{-1} \boldsymbol{e}_j \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \right) + \text{tr}\left( \boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \boldsymbol{e}_j^\top \right)\label{eq:7-8-3-3}\end{equation}

We compute the first term. Using the cyclic property of trace, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \boldsymbol{e}_j \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \right) = \text{tr}\left( \boldsymbol{e}_i^\top \boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1} \boldsymbol{e}_j \right) = (\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1})_{ij}\label{eq:7-8-3-4}\end{equation}

We compute the second term. Similarly using the cyclic property of trace, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \boldsymbol{e}_j^\top \right) = \text{tr}\left( \boldsymbol{e}_j^\top \boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{e}_i \right) = (\boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A})_{ji}\label{eq:7-8-3-5}\end{equation}

We transform the result of \eqref{eq:7-8-3-5}. $(\boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A})_{ji}$ equals the $(i, j)$ component of the transpose of the matrix $\boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A}$.

\begin{equation}(\boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A})_{ji} = ((\boldsymbol{Y}^{-1} \boldsymbol{X}^\top \boldsymbol{A})^\top)_{ij} = (\boldsymbol{A}^\top \boldsymbol{X} \boldsymbol{Y}^{-\top})_{ij}\label{eq:7-8-3-6}\end{equation}

When $\boldsymbol{A}$ is a general matrix, $\boldsymbol{Y} = \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}$ is not necessarily symmetric. Therefore $\boldsymbol{Y}^{-\top} \neq \boldsymbol{Y}^{-1}$.

We compute $\boldsymbol{Y}^\top = (\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X})^\top = \boldsymbol{X}^\top \boldsymbol{A}^\top \boldsymbol{X}$. This has the form where $\boldsymbol{A}$ is replaced by $\boldsymbol{A}^\top$.

Therefore the following holds.

\begin{equation}\boldsymbol{Y}^{-\top} = (\boldsymbol{Y}^\top)^{-1} = (\boldsymbol{X}^\top \boldsymbol{A}^\top \boldsymbol{X})^{-1}\label{eq:7-8-3-7}\end{equation}

Combining \eqref{eq:7-8-3-4} and \eqref{eq:7-8-3-6}, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} \right) = (\boldsymbol{A} \boldsymbol{X} \boldsymbol{Y}^{-1})_{ij} + (\boldsymbol{A}^\top \boldsymbol{X} \boldsymbol{Y}^{-\top})_{ij}\label{eq:7-8-3-8}\end{equation}

Substituting \eqref{eq:7-8-3-7}, we obtain the following.

\begin{equation}\text{tr}\left( \boldsymbol{Y}^{-1} \dfrac{\partial \boldsymbol{Y}}{\partial X_{ij}} \right) = (\boldsymbol{A} \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X})^{-1})_{ij} + (\boldsymbol{A}^\top \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A}^\top \boldsymbol{X})^{-1})_{ij}\label{eq:7-8-3-9}\end{equation}

Substituting into \eqref{eq:7-8-3-1}, we obtain the derivative with respect to component $X_{ij}$.

\begin{equation}\dfrac{\partial |\boldsymbol{Y}|}{\partial X_{ij}} = |\boldsymbol{Y}| \left[ (\boldsymbol{A} \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X})^{-1})_{ij} + (\boldsymbol{A}^\top \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A}^\top \boldsymbol{X})^{-1})_{ij} \right]\label{eq:7-8-3-10}\end{equation}

Writing in matrix form using the denominator layout, we obtain the following.

\begin{equation}\dfrac{\partial |\boldsymbol{Y}|}{\partial \boldsymbol{X}} = |\boldsymbol{Y}| \left( \boldsymbol{A} \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X})^{-1} + \boldsymbol{A}^\top \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A}^\top \boldsymbol{X})^{-1} \right)\label{eq:7-8-3-11}\end{equation}

Substituting $\boldsymbol{Y} = \boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}$, we obtain the final result.

\begin{equation}\dfrac{\partial |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}|}{\partial \boldsymbol{X}} = |\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X}| \left( \boldsymbol{A} \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A} \boldsymbol{X})^{-1} + \boldsymbol{A}^\top \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{A}^\top \boldsymbol{X})^{-1} \right)\label{eq:7-8-3-12}\end{equation}

Proof

In 7.8.2, set $\boldsymbol{A} = \boldsymbol{I}$ (the identity matrix). Since $\boldsymbol{I}$ is symmetric, the formula from 7.8.2 applies.

From 7.8.2, the following holds.

\begin{equation}\dfrac{\partial |\boldsymbol{X}^\top \boldsymbol{I} \boldsymbol{X}|}{\partial \boldsymbol{X}} = 2 |\boldsymbol{X}^\top \boldsymbol{I} \boldsymbol{X}| \boldsymbol{I} \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{I} \boldsymbol{X})^{-1}\label{eq:7-9-1-1}\end{equation}

Substituting $\boldsymbol{I} \boldsymbol{X} = \boldsymbol{X}$ and $\boldsymbol{X}^\top \boldsymbol{I} \boldsymbol{X} = \boldsymbol{X}^\top \boldsymbol{X}$, we obtain the following.

\begin{equation}\dfrac{\partial |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}} = 2 |\boldsymbol{X}^\top \boldsymbol{X}| \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1}\label{eq:7-9-1-2}\end{equation}

To differentiate $\log |\boldsymbol{X}^\top \boldsymbol{X}|$ with respect to $\boldsymbol{X}$, we apply the chain rule. The outer function is $\log$ and the inner function is $|\boldsymbol{X}^\top \boldsymbol{X}|$.

Writing out the chain rule with $f(u) = \log(u)$ and $g(\boldsymbol{X}) = |\boldsymbol{X}^\top \boldsymbol{X}|$, the following holds.

\begin{equation}\dfrac{\partial \log |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}} = \dfrac{1}{|\boldsymbol{X}^\top \boldsymbol{X}|} \dfrac{\partial |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}}\label{eq:7-9-1-3}\end{equation}

Substituting the result of \eqref{eq:7-9-1-2} into \eqref{eq:7-9-1-3}, we obtain the following.

\begin{equation}\dfrac{\partial \log |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}} = \dfrac{1}{|\boldsymbol{X}^\top \boldsymbol{X}|} \cdot 2 |\boldsymbol{X}^\top \boldsymbol{X}| \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1}\label{eq:7-9-1-4}\end{equation}

$\dfrac{1}{|\boldsymbol{X}^\top \boldsymbol{X}|}$ and $|\boldsymbol{X}^\top \boldsymbol{X}|$ cancel.

\begin{equation}\dfrac{\partial \log |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}} = 2 \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1}\label{eq:7-9-1-5}\end{equation}

We derive the relationship with the Moore-Penrose pseudoinverse. When $\boldsymbol{X}$ has full rank ($\text{rank}(\boldsymbol{X}) = N$), the left inverse is defined as follows.

\begin{equation}\boldsymbol{X}^{+} = (\boldsymbol{X}^\top \boldsymbol{X})^{-1} \boldsymbol{X}^\top\label{eq:7-9-1-6}\end{equation}

We compute the transpose of $\boldsymbol{X}^{+}$. Since the transpose of a product is $(\boldsymbol{P}\boldsymbol{Q})^\top = \boldsymbol{Q}^\top \boldsymbol{P}^\top$, we obtain the following.

\begin{equation}(\boldsymbol{X}^{+})^\top = ((\boldsymbol{X}^\top \boldsymbol{X})^{-1} \boldsymbol{X}^\top)^\top = (\boldsymbol{X}^\top)^\top ((\boldsymbol{X}^\top \boldsymbol{X})^{-1})^\top\label{eq:7-9-1-7}\end{equation}

$(\boldsymbol{X}^\top)^\top = \boldsymbol{X}$. Also, $\boldsymbol{X}^\top \boldsymbol{X}$ is a symmetric matrix, so its inverse is also symmetric. Therefore $((\boldsymbol{X}^\top \boldsymbol{X})^{-1})^\top = (\boldsymbol{X}^\top \boldsymbol{X})^{-1}$.

Combining \eqref{eq:7-9-1-7} with these properties, we obtain the following.

\begin{equation}(\boldsymbol{X}^{+})^\top = \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1}\label{eq:7-9-1-8}\end{equation}

Comparing \eqref{eq:7-9-1-5} and \eqref{eq:7-9-1-8}, we obtain the final result.

\begin{equation}\dfrac{\partial \log |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}} = 2 \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1} = 2 (\boldsymbol{X}^{+})^\top\label{eq:7-9-1-9}\end{equation}

Proof

We derive the relationship between $|\boldsymbol{X}^\top \boldsymbol{X}|$ and $|\boldsymbol{X}^{+}|$. We compute the determinant of $\boldsymbol{X}^{+} = (\boldsymbol{X}^\top \boldsymbol{X})^{-1} \boldsymbol{X}^\top$.

$\boldsymbol{X}^{+}$ is an $N \times M$ matrix, so when $N \neq M$ it is not a square matrix and the usual determinant is not defined. Therefore we consider $(\boldsymbol{X}^{+})^\top \boldsymbol{X}^{+}$.

From \eqref{eq:7-9-1-8} of 7.9.1, $(\boldsymbol{X}^{+})^\top = \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1}$.

Computing $(\boldsymbol{X}^{+})^\top \boldsymbol{X}^{+}$, we obtain the following.

\begin{equation}(\boldsymbol{X}^{+})^\top \boldsymbol{X}^{+} = \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1} \cdot (\boldsymbol{X}^\top \boldsymbol{X})^{-1} \boldsymbol{X}^\top\label{eq:7-9-2-1}\end{equation}

Since $(\boldsymbol{X}^\top \boldsymbol{X})^{-1} \cdot (\boldsymbol{X}^\top \boldsymbol{X})^{-1} = ((\boldsymbol{X}^\top \boldsymbol{X})^{-1})^2 = (\boldsymbol{X}^\top \boldsymbol{X})^{-2}$, we obtain the following.

\begin{equation}(\boldsymbol{X}^{+})^\top \boldsymbol{X}^{+} = \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-2} \boldsymbol{X}^\top\label{eq:7-9-2-2}\end{equation}

On the other hand, computing $\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top$, we obtain the following.

\begin{equation}\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top = (\boldsymbol{X}^\top \boldsymbol{X})^{-1} \boldsymbol{X}^\top \cdot \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1}\label{eq:7-9-2-3}\end{equation}

Simplifying the adjacent $\boldsymbol{X}^\top \boldsymbol{X}$ and $(\boldsymbol{X}^\top \boldsymbol{X})^{-1}$, we obtain the following.

\begin{equation}\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top = (\boldsymbol{X}^\top \boldsymbol{X})^{-1} \cdot \boldsymbol{X}^\top \boldsymbol{X} \cdot (\boldsymbol{X}^\top \boldsymbol{X})^{-1} = (\boldsymbol{X}^\top \boldsymbol{X})^{-1}\label{eq:7-9-2-4}\end{equation}

Taking the determinant of both sides of \eqref{eq:7-9-2-4}. $\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top$ is an $N \times N$ square matrix.

\begin{equation}|\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top| = |(\boldsymbol{X}^\top \boldsymbol{X})^{-1}|\label{eq:7-9-2-5}\end{equation}

The determinant of an inverse matrix is the reciprocal of the original determinant. That is, $|(\boldsymbol{X}^\top \boldsymbol{X})^{-1}| = \dfrac{1}{|\boldsymbol{X}^\top \boldsymbol{X}|}$.

Taking the logarithm of both sides, we obtain the following.

\begin{equation}\log |\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top| = \log \dfrac{1}{|\boldsymbol{X}^\top \boldsymbol{X}|} = -\log |\boldsymbol{X}^\top \boldsymbol{X}|\label{eq:7-9-2-6}\end{equation}

Therefore the following relation holds.

\begin{equation}\log |\boldsymbol{X}^\top \boldsymbol{X}| = -\log |\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top|\label{eq:7-9-2-7}\end{equation}

We consider differentiation with respect to $\boldsymbol{X}^{+}$. Viewing $\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top$ as a function of $\boldsymbol{X}^{+}$, it has the same form $\boldsymbol{Y}^\top \boldsymbol{Y}$ (with $\boldsymbol{Y} = (\boldsymbol{X}^{+})^\top$) as in 7.9.1.

From 7.9.1, $\dfrac{\partial \log |\boldsymbol{Y}^\top \boldsymbol{Y}|}{\partial \boldsymbol{Y}} = 2 \boldsymbol{Y} (\boldsymbol{Y}^\top \boldsymbol{Y})^{-1}$.

Setting $\boldsymbol{Y} = (\boldsymbol{X}^{+})^\top$, we have $\boldsymbol{Y}^\top = \boldsymbol{X}^{+}$ so $\boldsymbol{Y}^\top \boldsymbol{Y} = \boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top$.

From \eqref{eq:7-9-2-7}, applying the chain rule, we obtain the following.

\begin{equation}\dfrac{\partial \log |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}^{+}} = -\dfrac{\partial \log |\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top|}{\partial \boldsymbol{X}^{+}}\label{eq:7-9-2-8}\end{equation}

We differentiate $\log |\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top|$ with respect to $\boldsymbol{X}^{+}$. From \eqref{eq:7-9-2-4}, $\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top = (\boldsymbol{X}^\top \boldsymbol{X})^{-1}$, and by a computation similar to 7.2, we obtain the following.

\begin{equation}\dfrac{\partial \log |\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top|}{\partial \boldsymbol{X}^{+}} = 2 (\boldsymbol{X}^{+})^\top (\boldsymbol{X}^{+} (\boldsymbol{X}^{+})^\top)^{-1} = 2 (\boldsymbol{X}^{+})^\top (\boldsymbol{X}^\top \boldsymbol{X})\label{eq:7-9-2-9}\end{equation}

Substituting $(\boldsymbol{X}^{+})^\top = \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1}$ from \eqref{eq:7-9-1-8} of 7.9.1, we obtain the following.

\begin{equation}2 (\boldsymbol{X}^{+})^\top (\boldsymbol{X}^\top \boldsymbol{X}) = 2 \boldsymbol{X} (\boldsymbol{X}^\top \boldsymbol{X})^{-1} (\boldsymbol{X}^\top \boldsymbol{X}) = 2 \boldsymbol{X}\label{eq:7-9-2-10}\end{equation}

Substituting into \eqref{eq:7-9-2-8}, we obtain the following.

\begin{equation}\dfrac{\partial \log |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}^{+}} = -2 \boldsymbol{X}\label{eq:7-9-2-11}\end{equation}

In the denominator layout, the result has the same shape as the variable being differentiated. Since $\boldsymbol{X}^{+}$ is $N \times M$, the result must also be $N \times M$. Since $\boldsymbol{X}$ is $M \times N$, we take the transpose to obtain the final result.

\begin{equation}\dfrac{\partial \log |\boldsymbol{X}^\top \boldsymbol{X}|}{\partial \boldsymbol{X}^{+}} = -2 \boldsymbol{X}^\top\label{eq:7-9-2-12}\end{equation}

Proof

We consider cases based on the sign of the determinant. For an invertible matrix $\boldsymbol{X}$, either $\det(\boldsymbol{X}) > 0$ or $\det(\boldsymbol{X}) < 0$ ($\det(\boldsymbol{X}) = 0$ contradicts invertibility).

Case 1: $\det(\boldsymbol{X}) > 0$

Since $|\det(\boldsymbol{X})| = \det(\boldsymbol{X})$, the following holds.

\begin{equation}\log |\det(\boldsymbol{X})| = \log \det(\boldsymbol{X})\label{eq:7-10-1}\end{equation}

From 7.2, the derivative of $\log \det(\boldsymbol{X})$ is as follows.

\begin{equation}\dfrac{\partial \log \det(\boldsymbol{X})}{\partial \boldsymbol{X}} = \boldsymbol{X}^{-\top}\label{eq:7-10-2}\end{equation}

Therefore, when $\det(\boldsymbol{X}) > 0$, the following holds.

\begin{equation}\dfrac{\partial \log |\det(\boldsymbol{X})|}{\partial \boldsymbol{X}} = \boldsymbol{X}^{-\top}\label{eq:7-10-3}\end{equation}

Case 2: $\det(\boldsymbol{X}) < 0$

Since $|\det(\boldsymbol{X})| = -\det(\boldsymbol{X})$, the following holds.

\begin{equation}\log |\det(\boldsymbol{X})| = \log(-\det(\boldsymbol{X}))\label{eq:7-10-4}\end{equation}

We differentiate $\log(-\det(\boldsymbol{X}))$ with respect to $\boldsymbol{X}$. This is a composite function, so we apply the chain rule. The outer function is $\log$ and the inner function is $-\det(\boldsymbol{X})$.

The derivative of the outer function $f(u) = \log(u)$ is $f'(u) = \dfrac{1}{u}$. Substituting $u = -\det(\boldsymbol{X})$, we obtain the following.

\begin{equation}f'(-\det(\boldsymbol{X})) = \dfrac{1}{-\det(\boldsymbol{X})}\label{eq:7-10-5}\end{equation}

We compute the derivative of the inner function $g(\boldsymbol{X}) = -\det(\boldsymbol{X})$. From 7.1, $\dfrac{\partial \det(\boldsymbol{X})}{\partial \boldsymbol{X}} = \det(\boldsymbol{X}) \boldsymbol{X}^{-\top}$, so the following holds.

\begin{equation}\dfrac{\partial (-\det(\boldsymbol{X}))}{\partial \boldsymbol{X}} = -\det(\boldsymbol{X}) \boldsymbol{X}^{-\top}\label{eq:7-10-6}\end{equation}

Applying the chain rule and multiplying \eqref{eq:7-10-5} and \eqref{eq:7-10-6}, we obtain the following.

\begin{equation}\dfrac{\partial \log(-\det(\boldsymbol{X}))}{\partial \boldsymbol{X}} = \dfrac{1}{-\det(\boldsymbol{X})} \cdot (-\det(\boldsymbol{X}) \boldsymbol{X}^{-\top})\label{eq:7-10-7}\end{equation}

$\dfrac{1}{-\det(\boldsymbol{X})}$ and $(-\det(\boldsymbol{X}))$ cancel, yielding the following.

\begin{equation}\dfrac{\partial \log(-\det(\boldsymbol{X}))}{\partial \boldsymbol{X}} = \boldsymbol{X}^{-\top}\label{eq:7-10-8}\end{equation}

Therefore, when $\det(\boldsymbol{X}) < 0$ as well, the following holds.

\begin{equation}\dfrac{\partial \log |\det(\boldsymbol{X})|}{\partial \boldsymbol{X}} = \boldsymbol{X}^{-\top}\label{eq:7-10-9}\end{equation}

Combining \eqref{eq:7-10-3} and \eqref{eq:7-10-9}, regardless of the sign of the determinant, we always obtain the same result as long as $\det(\boldsymbol{X}) \neq 0$.

\begin{equation}\dfrac{\partial \log |\det(\boldsymbol{X})|}{\partial \boldsymbol{X}} = \boldsymbol{X}^{-\top}\label{eq:7-10-10}\end{equation}