Thanks for creating lots of exercise discussions!

Here's a problem I stumbled upon: in LaTeX, { and } are control characters, so

$$f_! (A) := { y \in Y | \exists a \in A \text{ such that } f a = y }$$

merely produces

$$f_! (A) := { y \in Y | \exists a \in A \text{ such that } f a = y }$$

In this installation of MathJax the solution is to use \{ and \}. For example:

$$f_! (A) := \\{ y \in Y | \exists a \in A \text{ such that } f a = y \\}$$

produces

$$f_! (A) := \{ y \in Y | \exists a \in A \text{ such that } f a = y \}$$

I'll repair these and other problems when I meet them. Starting on Thursday I'll go through all the exercises in Chapter 1 and "announce" them.