# Derivative of Kronecker product of vector with itself

I'm struggling with the following problem. Suppose $$\pmb{x}$$ and $$\pmb{y}$$ are vectors of the same length and $$\pmb{y}$$ is not a function of $$\pmb{x}$$. What is the following derivative?

$$\frac{\partial}{\partial \pmb{x}} (\pmb{y} - \pmb{x}) \otimes (\pmb{y} - \pmb{x})$$

My thought was to write use $$\pmb{z} = \pmb{y} - \pmb{x}$$ and $$\pmb{f} = \pmb{z} \otimes \pmb{z}$$ and derive first:

\begin{align} d\pmb{f} &= ((d\pmb{z}) \otimes \pmb{z}) + (\pmb{z} \otimes (d\pmb{z})) \\ &= (\pmb{I} \otimes \pmb{z})d\pmb{z} + (\pmb{z} \otimes \pmb{I})d\pmb{z} \\ &= ((\pmb{I} \otimes \pmb{z}) + (\pmb{z} \otimes \pmb{I}))d\pmb{z} \\ \frac{\partial \pmb{f}}{\partial \pmb{z}} &= (\pmb{I} \otimes \pmb{z}) + (\pmb{z} \otimes \pmb{I}) \end{align}

and then obtain by chain rule:

$$\frac{\partial}{\partial \pmb{x}} (\pmb{y} - \pmb{x}) \otimes (\pmb{y} - \pmb{x}) = -\left( (\pmb{I} \otimes (\pmb{y} - \pmb{x})) + ((\pmb{y} - \pmb{x}) \otimes \pmb{I}) \right)$$

Which seems sensble. However, this is part of a Hessian I am deriving, and it's corresponding transpose element I derived to be:

$$-2\left(\pmb{I} \otimes (\pmb{y} - \pmb{x})\right)$$

Which is very similar but not the same. Am I missing something obvious?

• This doesn't really call for the chain rule. Just substitute $(dz\longrightarrow -dx)$ into the differential expression, and that leads directly to the given gradient. – lynn Feb 14 at 19:52

Let's clear out some definitions first.

If $$f:z \to f(x)$$ is a matrix valued function and there is a function $$D_f$$ such that $$f(z+h) = f(z) + D_f(z,h) + o(\|h\|)$$ Then $$D_f$$ is the differential of $$f$$. If there exists a matrix valued function $$A(z)$$ such that $$D_f(z,h) = A(z) h$$ Then $$A(z)$$ is the derivative of $$f$$.

(This is sometimes called the first identification theorem; see for instance Magnus and Neudecker, 1999).

In the case at hand, we have $$f(z+h) = (z+h)\otimes (z+h) = \underbrace{z\otimes z}_{f(z)} + \underbrace{(h\otimes z) + (z\otimes h)}_{D_f(z,h)} + \underbrace{h\otimes h}_{o(\|h\|)}$$

So, by the definition $$D_f(z,h) = (h\otimes z) + (z\otimes h)$$ is the differential of $$z\otimes z$$. Now we can use the identification theorem to say that, since $$D_f(z,h) = \big[(I\otimes z) + (z \otimes I)\big]h = A(z) h$$ the matrix $$A(z) = (I\otimes z) + (z \otimes I)$$ is the derivative of $$z \otimes z$$.

So in the case at hand, the same reasoning brings you to the correct derivative $$\frac{\partial}{\partial x}(y-x)\otimes (y-x) = \big(I\otimes(x-y)\big) + \big((x-y) \otimes I\big)$$ which is what you found.

Check the other half of the Hessian: there has to be something wrong there!

• Thanks! The other half had a mistake indeed. I also didn't mention a duplication matrix I use to take the derivative to a half-vectorization (didn't think it would matter). I got for the other half $-2 \left(\left(\pmb{y} - \pmb{x} \right)^{\top} \otimes \pmb{I}\right) \pmb{D}$ and for this part $- \pmb{D}^{\top} \left( (\pmb{I} \otimes (\pmb{y} - \pmb{x})) + ((\pmb{y} - \pmb{x}) \otimes \pmb{I}) \right)$. For some reason, these turn out to be each-others transposes numerically. I don't really get how, magic I guess :) But it will be sufficient – Sacha Epskamp Feb 14 at 21:09

The gradient that you found is correct. I re-derive it here to make this answer self-contained...

First note that the Kronecker product of two vectors can be expanded in two ways: $$a\otimes b = (I_a\otimes b)\,a = (a\otimes I_b)\,b$$ where $$I_a$$ is the identity matrix whose dimensions are compatible with the $$a$$ vector, while $$I_b$$ is compatible with the $$b$$ vector.

Define two new vectors \eqalign{ z &= x-y \quad\implies dz = dx\cr f &= z\otimes z \cr } Then use the Kronecker expansion to calculate the differential and gradient of $$f$$. \eqalign{ df &= z\otimes dz + dz\otimes z = (z\otimes I + I\otimes z)\,dx \cr G=\frac{\partial f}{\partial x} &= (z\otimes I + I\otimes z) = (x-y)\otimes I + I\otimes(x-y) \cr } Let $$e_k$$ denote the $$k^{th}$$ column of the $$I$$ matrix and $$w={\rm vec}(I)$$.
Use these to vectorize the $$G$$ matrix. \eqalign{ G &= (z\otimes I + I\otimes z), \quad M = \pmatrix{I\otimes e_1\cr I\otimes e_2\cr\vdots\cr I\otimes e_n} \cr g &= {\rm vec}(G) = \Big(M + w\otimes I\Big)\,z \cr } Now find the differential and gradient of the $$g$$ vector. \eqalign{ dg &= \Big(M + w\otimes I\Big)\,dx \cr H = \frac{\partial g}{\partial x} &= \Big(M + w\otimes I\Big) \cr } So that's the hessian in matrix form. The true Hessian is a 3rd order tensor.