# Derivative with Reparameterisation Trick

Below is some steps for differentiating a function wrt a set of parameters $$\phi$$ using the "reparameterisation trick" (Kingma & Welling 2013).

However after applying the derivative as follows I cannot follow how the underlined portion of the derivative came about. Could someone simplify or explain the origin of this term?

• Do you understand where the second term on that line comes from? – jbowman Mar 11 at 14:43
• Yes. That is chain rule. To be honest I only would have ended up with the second term and not included the first term – pche8701 Mar 11 at 22:21

While you are right on the second term and the use of the chain rule, your distribution $$q$$ also directly depends on $$\phi$$ as made explicit by the subscript $$q_\mathbf{\phi}$$. That's why I prefer the notation $$q(\cdot \, ;\phi)$$, everything is more clear.
Denoting from the beginning: $$\theta = f(\epsilon \, ; \phi)$$ and $$g = \log q(\theta \, ; \phi)$$, we have using Leibniz's notation:
\begin{align*} \frac{\partial g(\phi, \theta)}{\partial \phi} = \left. \frac{\partial g(\phi, \theta)}{\partial \phi} \right|_{\theta = f(\epsilon \, ; \phi)} + \frac{\partial g(\phi, \theta)}{\partial \theta} \frac{\partial f(\epsilon \, ; \phi)}{\partial \phi} \; \textrm{,} \end{align*}
where the derivative of the first term on the right-hand side is computed with $$\theta$$ fixed