# Difference between revisions of "Hardy hierarchy"

The Hardy hierarchy, named after G. H. Hardy, is a family of functions $$(H_\alpha:\mathbb N\rightarrow\mathbb N)_{\alpha<\mu}$$ where $$\mu$$ is a large countable ordinal such that a fundamental sequence is assigned for each limit ordinal less than $$\mu$$. The Hardy hierarchy is defined as follows:

• $$H_0(n)=n$$
• $$H_{\alpha+1}(n)=H_\alpha(n+1)$$
• $$H_\alpha(n)=H_{\alpha[n]}(n)$$ if and only if $$\alpha$$ is a limit ordinal,

where $$\alpha[n]$$ denotes the $$n$$th element of the fundamental sequence assigned to the limit ordinal $$\alpha$$

Every nonzero ordinal $$\alpha<\varepsilon_0=\min\{\beta|\beta=\omega^\beta\}$$ can be represented in a unique Cantor normal form $$\alpha=\omega^{\beta_{1}}+ \omega^{\beta_{2}}+\cdots+\omega^{\beta_{k-1}}+\omega^{\beta_{k}}$$ where $$\alpha>\beta_1\geq\beta_2\geq\cdots\geq\beta_{k-1}\geq\beta_k$$.

If $$\beta_k>0$$ then $$\alpha$$ is a limit and we can assign to it a fundamental sequence as follows

$$\alpha[n]=\omega^{\beta_{1}}+ \omega^{\beta_{2}}+\cdots+\omega^{\beta_{k-1}}+\left\{\begin{array}{lcr} \omega^\gamma n \text{ if } \beta_k=\gamma+1\\ \omega^{\beta_k[n]} \text{ if } \beta_k \text{ is a limit.}\\ \end{array}\right.$$

If $$\alpha=\varepsilon_0$$ then $$\alpha[0]=0$$ and $$\alpha[n+1]=\omega^{\alpha[n]}$$.

Using this system of fundamental sequences we can define the Hardy hierarchy up to $$\varepsilon_0$$.

For $$\alpha<\varepsilon_0$$ the Hardy Hierarchy relates to the fast-growing hierarchy as follows

$$H_{\omega^\alpha}(n)=f_\alpha(n)$$

and at $$\varepsilon_0$$ the Hardy hierarchy "catches up" to the fast-growing hierarchy i.e.

$$f_{\varepsilon_0}(n-1) ≤ H_{\varepsilon_0}(n) ≤ f_{\varepsilon_0}(n+1)$$ for all $$n ≥ 1$$.

There are much stronger systems of fundamental sequences you can see on the following pages:

The Hardy hierarchy has the following property $$H_{\alpha+\beta}(n)=H_\alpha(H_\beta(n))$$.

## References

Hardy,G.H. A theorem concerning the infinite cardinal numbers. Quarterly Journal of Mathematics (1904) vol.35 pp.87–94