Fast-growing hierarchy ($F_0(n)=n+1,F_{\alpha +1}(n)=F_{\alpha }^{n+1}(n),F_{\lambda }(n)=F_{\lambda [n]}(n)$)

A fast-growing hierarchy indexes number-theoretic functions by an effective system of ordinal notations. Successor stages iterate the preceding function, while limit stages diagonalize along a chosen fundamental sequence of a limit ordinal. These choices are part of the definition. With the standard Cantor normal form sequences below epsilon zero, each fixed $F_{\alpha }$ for $\alpha <{\epsilon }_0$ is provably total in Peano arithmetic, but the uniform endpoint $F_{{\epsilon }_0}$ is not. Every function provably total in Peano arithmetic is eventually bounded by some lower level. These classification results are proved in epub.ub.uni-muenchen.de/3843/1/3843.pdf .

For the standard fundamental sequences of limit ordinals below epsilon zero, the index order corresponds to eventual, rather than necessarily pointwise, growth. For example, under $\omega [n]=n+1$ and successor iteration $n+1$ times, $F_{\omega }(1)=F_2(1)=7$ whereas $F_3(1)=2047$. Limit stages are diagonal choices, so a larger index can give a smaller value at small arguments.