I've had a think and, having thunk, I've removed the closed form, but also amended the description to make it clear that such a thing exists.

Appreciate the feedback!

It's a very valid question and represents that I'm in two minds over this. In line with exactly your point, the original description didn't include a closed form. However, the initial comments when I put this up as a potential Kata suggested that just having the recursive definition looked confusing, so I subsequently altered it to include one of the direct methods of computation knowing that it took away from the original spirit.

I guess I just need to decide whether to revert to my original proposal, but, as this is my first attempt at a Kata, I'm more than happy to hear both what people think...

** nods head ** I know, and I completely understand. I tried both styles side by side and I genuinely prefer that L/KaTeX style for mathematical expressions...

Very much obliged on the suggestion of KaTeX: I did wonder if methematical notation was possible, but don't know LaTeX. I've updated the look and feel of the formulae side of things to use KaTeX and also included a non-recursive definition of the swinging factorial function which looks a bit less ... technical ...

I've added a small note to the description to try and clarify this...

Fair question; part of the Kata was to look up a straightforward method to calculate the values (hence the references in the hints). Perhaps I should make this a bit more explicit in the description..?