# Matrix calculus: Second-order derivative matrix using chain rule

Let's consider three sets of vectors, dependent on each other as follows: $\textbf{A}=\textbf{A}(\textbf{B})$, $\textbf{B}=\textbf{B}(\textbf{C})$

Using the chain rule, $\dfrac{d\textbf{A}}{d\textbf{C}}=\dfrac{d\textbf{A}}{d\textbf{B}}\dfrac{d\textbf{B}}{d\textbf{C}}$

How can one expand this rule to calculate the second order derivative matrix $\dfrac{d^2\textbf{A}}{d\textbf{C}^2}$ if $\dfrac{d^2\textbf{A}}{d\textbf{B}^2}, \dfrac{d^2\textbf{B}}{d\textbf{C}^2}$ are also known?

I was thinking something like $\dfrac{d^2\textbf{A}}{d\textbf{C}^2}= \dfrac{d\textbf{B}}{d\textbf{C}}^T \dfrac{d^2\textbf{A}}{d\textbf{B}^2} \dfrac{d\textbf{B}}{d\textbf{C}} + \dfrac{d\textbf{A}}{d\textbf{B}} \dfrac{d^2\textbf{B}}{d\textbf{C}^2}$ that reminds of the multivariable function second-order chain rule, but I don't think it holds.

Thanks for all the tips!

edit: I am using the Python NumPy package, so I can add over any matrix axes using tensordot.

EDIT: Using einsum of the NumPy package, one can write:

dAdC=np.einsum('ik,km->im',dAdB,dBdC)


for the first (small d) and second order (capital D) derivatives, respectively.

• I'm not sure your expression is correct because the second order derivatives should form a 3-tensor (See here for example. It isn't clear how you would combine this with the 2-tensors given by, e.g. $dA/dB$. – Paul Aljabar Dec 31 '17 at 18:33

Depending on how the tensors are combined, I think your expression might make sense now.

I've used $x$, $y$ and $z$ for $C$, $B$ and $A$ respectively in the work below, hope it makes sense:

Let $x\in \mathbb{R}^m$ and $y \in \mathbb{R}^n$. Let $y$ depend on $x$ so we can find the Jacobian of $y$ as a function of $x$.

$$J_x y \in \mathbb{R}^{n \times m} \qquad \left. J_{x} y \right|_{i,j} = \frac{\partial y_{i}}{\partial x_{j}}$$

where $J|_{i,j}$ denotes the $(i,j)$ element of the Jacobian.

Let $z \in \mathbb{R}^{k}$ be a function of $y$. We can also find the Jacobian of $z$ as a function of $y$.

$$J_y z \in \mathbb{R}^{k \times n} \qquad \left. J_{y} z \right|_{t,i} = \frac{\partial z_{t}}{\partial y_{i}}$$

We can view $z$ as a function of $x$ by composition

$$x \rightarrow y \rightarrow z \qquad \qquad \mathbb{R}^{m} \rightarrow \mathbb{R}^{n} \rightarrow \mathbb{R}^{k}$$

The Jacobian of $z$ as a function of $x$:

$$J_x z \in \mathbb{R}^{k \times m} \qquad \left. J_{x} z \right|_{t,j} = \frac{\partial z_{t}}{\partial x_{j}}$$

The term

$$\frac{\partial z_{t}}{\partial x_{j}}$$

Can be written as a sum

$$\frac{\partial z_{t}}{\partial x_{j}} = \sum_{i=1}^{n} \frac{\partial z_{t}}{\partial y_{i}} \frac{\partial y_{i}}{\partial x_{j}}$$

Let's drop the summation and use Einstein notation, i.e. we assume summation over indices that are repeated, so we can write

$$\frac{\partial z_{t}}{\partial x_{j}} = \frac{\partial z_{t}}{\partial y_{i}} \frac{\partial y_{i}}{\partial x_{j}}$$

This basically represents matrix multiplication. The Jacobians are matrices (2-tensors) and we have

$$J_{x} z = J_{y} z \; \; J_{x} y$$

What about the Hessian? - is there a chain rule for Hessians?

The Hessian of $y$ as a function of $x$ is

$$H_{x} y = \nabla_{x} J_{x} y$$

It is a 3-tensor $$\left. H_{x} y \right\vert_{q,i,j} = \frac{\partial}{\partial x_{q}} J_{x} y \vert_{i,j}$$

$$\left. H_{x} y \right\vert_{q,i,j} = \frac{\partial}{\partial x_{q}} \frac{\partial y_{i}}{\partial x_{j}} = \frac{\partial^{2} y_{i}}{\partial x_{q} \, x_{j}}$$

The Hessian of $z$ as a function of $y$ is

$$\left. H_{y} z \right\vert_{r,t,i} = \frac{\partial}{\partial y_{r}} \frac{\partial z_{t}}{\partial y_{i}} = \frac{\partial^{2} z_{t}}{\partial y_{r} \, y_{i}}$$

What about the Hessian of $z$ as a function $x$?

$$\left. H_{x} z \right\vert_{s,t,j} = \frac{\partial}{\partial x_{s}} \frac{\partial z_{t}}{\partial x_{j}} = \frac{\partial^{2} z_{t}}{\partial x_{s} \, x_{j}}$$

We can use the expression derived earlier for $\partial z_{t}/\partial x_{j}$ which is an element in the Jacobian of $z$ with respect to $x$:

$$\left. H_{x} z \right\vert_{s,t,j} = \frac{\partial}{\partial x_{s}} \left[ \frac{\partial z_{t}}{\partial y_{i}} \frac{\partial y_{i}}{\partial x_{j}} \right]$$

Remembering that we are using the convention where repeated indices denote indices over which we should sum. The evaluates with the product rule to the following

\begin{aligned} \left. H_{x} z \right\vert_{s,t,j} & = \frac{\partial}{\partial x_{s}} \left[ \frac{\partial z_{t}}{\partial y_{i}} \frac{\partial y_{i}}{\partial x_{j}} \right] \\ &= \frac{\partial z_{t}}{\partial y_{i}} \; \frac{\partial}{\partial x_{s}} \frac{\partial y_{i}}{\partial x_{j}} + \frac{\partial}{\partial x_{s}} \frac{\partial z_{t}}{\partial y_{i}} \; \frac{\partial y_{i}}{\partial x_{j}} \\ &= \frac{\partial z_{t}}{\partial y_{i}} \, \frac{\partial^{2} y_{i}}{\partial x_{s} x_{j}} + \frac{\partial}{\partial x_{s}} \frac{\partial z_{t}}{\partial y_{i}} \, \frac{\partial y_{i}}{\partial x_{j}} \end{aligned}

The first part of the second term contains components of all three variables, let's focus on it

$$\frac{\partial}{\partial x_{s}} \frac{\partial z_{t}}{\partial y_{i}}$$

Assuming equality of mixed partials, we can write it as

$$\frac{\partial}{\partial y_{i} } \frac{\partial z_{t}}{\partial x_{s}}$$

\begin{aligned} \frac{\partial}{\partial y_{i} } \frac{\partial z_{t}}{\partial x_{s}} &= \frac{\partial}{\partial y_{i} } \left[ \frac{\partial z_{t}}{\partial y_{u}} \frac{\partial y_{u}}{\partial x_{s}} \right] \\ &= \frac{\partial}{\partial y_{i} } \frac{\partial z_{t}}{\partial y_{u}} \; \frac{\partial y_{u}}{\partial x_{s}} + \frac{\partial z_{t}}{\partial y_{u}} \; \frac{\partial}{\partial y_{i} } \frac{\partial y_{u}}{\partial x_{s}} \\ &= \frac{\partial^{2} z_{t}}{\partial y_{i} y_{u}} \; \frac{\partial y_{u}}{\partial x_{s}} + \frac{\partial z_{t}}{\partial y_{u}} \; \frac{\partial}{\partial y_{i} } \frac{\partial y_{u}}{\partial x_{s}} \\ &= \frac{\partial^{2} z_{t}}{\partial y_{i} y_{u}} \; \frac{\partial y_{u}}{\partial x_{s}} + \frac{\partial z_{t}}{\partial y_{u}} \; \frac{\partial}{\partial x_{s} } \frac{\partial y_{u}}{\partial y_{i} } \\ &= \frac{\partial^{2} z_{t}}{\partial y_{i} y_{u}} \; \frac{\partial y_{u}}{\partial x_{s}} + \frac{\partial z_{t}}{\partial y_{y}} \; \frac{\partial}{\partial x_{s} } \delta_{i,u} \\ &= \frac{\partial^{2} z_{t}}{\partial y_{i} y_{u}} \; \frac{\partial y_{u}}{\partial x_{s}} + \frac{\partial z_{t}}{\partial y_{u}} \; 0 \\ &= \frac{\partial^{2} z_{t}}{\partial y_{i} y_{u}} \; \frac{\partial y_{u}}{\partial x_{s}} \end{aligned}

Now replace this expression into the second term of the earlier equation for the Hessian of $z$ with respect to $x$

\begin{aligned} \left. H_{x} z \right\vert_{s,t,j} &= \frac{\partial z_{t}}{\partial y_{i}} \, \frac{\partial^{2} y_{i}}{\partial x_{s} x_{j}} + \frac{\partial}{\partial x_{s}} \frac{\partial z_{t}}{\partial y_{i}} \, \frac{\partial y_{i}}{\partial x_{j}} \\ &= \frac{\partial z_{t}}{\partial y_{i}} \, \frac{\partial^{2} y_{i}}{\partial x_{s} x_{j}} + \frac{\partial^{2} z_{t}}{\partial y_{i} y_{u}} \; \frac{\partial y_{u}}{\partial x_{s}} \, \frac{\partial y_{i}}{\partial x_{j}} \end{aligned}

Now we can replace the partial derivative notation with the Jacobian/Hessian notation to get an expression for the Hessian of $z$ as a function of $x$ in terms of the intermediate Jacobians/Hessians:

\begin{aligned} \left. H_{x} z \right\vert_{s,t,j} &= \frac{\partial z_{t}}{\partial y_{i}} \, \frac{\partial^{2} y_{i}}{\partial x_{s} x_{j}} + \frac{\partial^{2} z_{t}}{\partial y_{i} y_{u}} \; \frac{\partial y_{u}}{\partial x_{s}} \, \frac{\partial y_{i}}{\partial x_{j}} \\ &= \left. J_{y} z \right|_{t,i} \, \left. H_{x} y \right|_{s,i,j} + \left. H_{y} z \right|_{i,t,u} \; \left. J_{x} y \right|_{{u,s}} \, \left. J_{x} y \right|_{i,j} \end{aligned}

So for the first term:

$$\left. J_{y} z \right|_{t,i} \, \left. H_{x} y \right|_{s,i,j}$$ we evaluate the tensor product along the second dimension of the Jacobian of $z$ and the second dimension of the Hessian of $y$. This is because the shared index $i$ appears second for $J_{y} z$ and second for $H_{x} y$. The Numpy tensordot function accepts an argument to set which dimensions to carry out the product on.

For the second term $$\left. H_{y} z \right|_{i,t,u} \; \left. J_{x} y \right|_{{u,s}} \, \left. J_{x} y \right|_{i,j}$$ There are two tensor products and the shared indices indicate over which dimensions to take the products.

• I've made a few changes and clarifications as the original answer had some pure LaTeX commands and some gaps in the description. Feedback welcome. – Paul Aljabar Jan 1 '18 at 3:32
• Thanks for the analysis, in the end I performed the chain rule using indices inside loops – whirlwind Jan 2 '18 at 13:59
• @whirlwind , No problem. Do you think the loop implementation you performed matches the analysis? – Paul Aljabar Jan 2 '18 at 14:41