The answer is a power tower and requires a language to do bignums. Probably a difficulty >8.

You’ll need to do a good job of explaining it, as after googling I have no idea what it means

The Ackerman function is a very simple recursive function. It is of theoretical interest in mathematics, but only in ways that do not concern you. (I *think*.)

If you must know:

- It is a computable function, which basically means that you (yes,
*you*) can write a program that computes its output for any given input, but - It is not primitive recursive, which basically means that
`for`

-loops as a looping construct are not enough to be able to express it: you also need`while`

-loops. - For some time in the past it was unknown whether any such functions – computable but not primitive recursive – exist. The Ackerman function, being an example of one, is proof that they do.

I need more information about your confusion to be able to explain more.

To calculate Ackermann 4,2 requires bignums. The task would be both to calculate 4,2 *and* implement a simple bignums library.

So, @MatthijsBlom , I’m not confused (well, no more than usual.)