For 2019, I’m trying an experiment: every couple of weeks, writing a post about a mathematical object that a) I don’t know much about and b) is named after somebody. These posts are a trial run - let me know how you find them!

The chief use of the Ackermann function, these days, is to show you can generate REALLY BIG numbers from small arguments. However, it plays an important role in computing theory: it was one of the first, and simplest, non-primitive-recursive functions.

### So what is it?

A primitive recursive function is defined using compositions and primitive recursion - which, in itself, sounds rather recursive. However, primitive recursion has a technical definition, relying on five axioms:

1. The constant function $0$ (taking no arguments) is primitive recursive.
2. The one-argument successor function $S(x) = x+1$ is primitive recursive.
3. The projection function $P_i^n(x_1, x_2, … x_n) = x_i$ is primitive recursive.
4. If a primitive recursive function $f$ takes $k$ arguments, and the primitive recursive functions $g_1, g_2, … g_k$ each take $m$ arguments, then their composition $f(g_1(x_1,…x_m), g_2(x_1,…x_m), … g_k(x_1,…x_m))$ is primitive recursive.
5. If $f$ is primitive recursive and takes $k$ arguments, and $g$ is primitive recursive and takes $k+2$ arguments, then the function $h$ (taking $k+1$ arguments) is the primitive recursion of $f$ and $g$ if:
• $h(0, x_1, … x_k) = f(x_1, … x_k)$ and;
• $h(y+1, x_1, … x_k) = g(y, h(y, x_1, … x_k), x_1, … x_k)$

### *Looks to other crew member* So what is it?

OK: primitive recusrive functions are ones that can be made by:

• adding 1 to a number
• repeating a function a fixed number of times (like a for loop)
• shuffling the arguments, or
• composing such functions with each other.

And what’s the point? Well, primitive recursive functions are guaranteed to halt - they can never get into an infinite loop. (If the restriction on how many times the function can be called is lifted, things become Turing complete - and you can’t tell whether an arbitrary Turing complete function will terminate or not.)

### The Ackermann Function

The Ackermann function (or, in this form, the Ackermann-Péter function), is given by:

• $A(0,n) = n+1$
• $A(m,0) = A(m-1, 1)$
• $A(m,n) = A(m-1, A(m, n-1) )$ if $m$ and $n$ are both positive.

That doesn’t seem too outrageous: for example, we can work out $A(1,1)$ without too much trouble:

Using the last definition, $A(1,1) = A(0, A(1,0) )$.

We need to work out $A(1,0)$, using the second definition, which gives $A(1,0) = A(0,1) = 2$ (using the first definition).

So, we have $A(1,1) = A(0, 2) = 3$. That’s not a big number!

However, as $m$ and $n$ increase, the Ackermann function increases dramatically - $A(4,2)$, for example, is around $2^{65536}$, or nearly $10^{20000}$.

It does, however, always terminate - each $A(m,n)$ is defined either on its own terms (if $m=0$) or in terms of “smaller” Ackermann functions - any functions it relies on have at least one smaller argument, and no bigger ones.

### But it’s not primitive recursive

I’m writing this with a raging headache and, I have to concede, a flimsy understanding of the details of the proof. My understanding of it goes as follows:

For every primitive recursive function $f$, there is an integer, $t$, such that $f(x_1,…, x_n) < A(t, \max(\{x_1,…,x_n\}))$.

This means that $A$ cannot be primitive recursive, or else $A(t,t) < A(t,t)$ - which is a contradiction. In short, $A$ grows more quickly than any primitive recursive function possibly can!

The Ackermann function is a lovely example (to me) of complexity developing from very simple rules - but its mathematical importance comes from showing you can have a function that’s guaranteed to halt, but that isn’t primitive recursive.

* Edited 2019-02-09 to change category