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Goodstein sequence
TypeCombinatorial
Growth rate$$f_{\varepsilon_0}(n)$$

A Goodstein sequence is a certain class of integer sequences Gk(n) that give rise to a quickly growing function that eventually dominates all recursive functions which are provably total in Peano arithmetic, but is itself provably total in PA + "$$\varepsilon_0$$ is well-ordered".

## Definition

Suppose we write a nonnegative integer n as a sum of powers of k, then we write the k exponents themselves as similar sums of powers, repeating this process until we get all topmost exponents less than k. For example, we can write 100 as 26 + 25 + 22 = 222 + 2 + 222 + 1 + 22. We call this the base-k hereditary representation as n.

In the above representation of 100, we can "bump" the base 2 up by one, forming a different number 333 + 3 + 333 + 1 + 33. We write this larger number as B(100), and in general, B[b](n) means finding the base-b hereditary representation of n and bumping the base.

Now we define the recursive sequence G0(n) = n and Gk(n) = B[k + 1](Gk - 1(n)) - 1. In other words, as k increases, we are repeatedly bumping the base and subtracting one:

G0(100) = 100 = 222 + 2 + 222 + 1 + 22
G1(100) = B(100) - 1 = 333 + 3 + 333 + 1 + 33 - 1 = 228,767,924,549,636
G2(100) = B(G1(100)) - 1 = 444 + 4 + 444 + 1 + 2 x 42 + 2 x 4 + 2 x 1 - 1 = 3.486030062 x 10156

This rapidly growing sequence is known a Goodstein sequence. Surprisingly, for all values of n, Gk(n) eventually peaks, declines, and returns to zero. This fact is known as Goodstein's theorem. Even more surprisingly, it can be shown that Goodstein's theorem cannot be proved in Peano arithmetic.

## Unprovability

Let G(n) be the number of steps it takes for the Goodstein sequence starting with n to terminate (i.e. reach zero). Formally, G(n) as the smallest k for which Gk(n) = 0. G(n) is an extremely fast-growing function. Note that some authors define G(n) as the length of the Goodstein sequence starting with n, which increases the outputs of the function by 1 because in this case the first step (G0(n)) is included.

Define $$R^\omega_b (n)$$ to be the ordinal obtained by writing n in base-b hereditary notation, then replacing every instance of b by $$\omega$$, for instance

$R^\omega_2 (100) = R^\omega_2 (2^{2^2 + 2} + 2^{2^2 + 1} + 2^2) = \omega^{\omega^\omega + \omega} + \omega^{\omega^\omega + 1} + \omega^\omega$

Then $$G(n) = H_{R^\omega_2 (n)} (3) - 3$$ in the Hardy hierarchy, which shows that $$G(2 \uparrow\uparrow n)$$ is $$H_{\varepsilon_0} (n)-3$$. So, it grows as fast as Jonathan Bowers' tetrational arrays. Since this function grows faster than any function provably recursive in Peano arithmetic, Goodstein's theorem is not provable in Peano arithmetic.

## Values and bounds of $$G(n)$$

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