# The Living Thing / Notebooks : Putting the funk in functions (Functional equations)

Miscellaneous tricks with functions. The purer side of the functional wrangling which gets you, e.g. variational approximation.

## Dan Piponi’s functional logarithms

Nice hack, Dan Piponi — Logarithms and exponentials of functions:

A popular question in mathematics is this: given a function $$f$$, what is its “square root” $$g$$ in the sense that $$g(g(x))=f(x)$$. […] I want to approach the problem indirectly. When working with real numbers we can find square roots, say, by using $$\sqrt{x}=\exp\left(\frac{1}{2}\log x\right)$$. I want to use an analogue of this for functions. So my goal is to make sense of the idea of the logarithm and exponential of a formal power series as composable functions.

## Tom Leinster’s course

Tom Leinster, is teaching a course on functional equations (course notes here):

Today was a warm-up, focusing on Cauchyâ€™s functional equation: which functions $$f: \mathbb{R} \to \mathbb{R}$$ satisfy

\begin{equation*} f(x + y) = f(x) + f(y) \,\,\,\, \forall x, y \in \mathbb{R}? \end{equation*}

He goes on to talk about Shannon entropy from a functional equation perspective, which is a refreshing derivation.