What is the name, meaning and function of the circle(s) in this composition of functions?

Question

This is the S-DES encryption algorithm. I don't recognize this character.

Sidebar: How can I write this in LaTeX/MathJax?

Answer 1

The circles are usually read "of", and they refer to function composition.

For two functions $f(x)$ and $g(x)$: $(f \circ g)(x) = f(g(x))$

So $\text{IP}^{-1} \circ f_{K_2} \circ \text{SW} \circ f_{K_1} \circ \text{IP} = \text{IP}^{-1} ( f_{K_2} ( \text{SW} ( f_{K_1} ( \text{IP}))))$.

Answer 2

It's a circle, denoted by \circ. For example,

$$f \circ g \circ h.$$

And then your given example looks like

$$\rm IP^{-1} \circ f_{K_2} \circ SW \circ f_{K_1} \circ IP.$$

And in the future, you should check out detexify for your symbol-finding needs (and/or google).