A proof of strongly uniform termination for Gödel's $T$ by methods from local predicativity

Abstract.

We estimate the derivation lengths of functionals in Gödel's system $T$ of primitive recursive functionals of finite type by a purely recursion-theoretic analysis of Schütte's 1977 exposition of Howard's weak normalization proof for $T$. By using collapsing techniques from Pohlers' local predicativity approach to proof theory and based on the Buchholz-Cichon and Weiermann 1994 approach to subrecursive hierarchies we define a collapsing f unction ${\cal D}:T\to \omega$ so that for (closed) terms $a,b$ of Gödel's $T$ we have: If $a$ reduces to $b$ then $\omega>{\cal D}(a)>{\cal D}(b).$ By one uniform proof we obtain as corollaries: A derivation lengths classification for functionals in $T$, hence new proof of strongly uniform termination of $T$. A new proof of the Kreisel's classific ation of the number-theoretic functions which can be defined in $T$, hence a classification of the provably total functions of Peano Arithmetic. A new proof of Tait's results on weak normalization for $T$. A new proof of Troelstra's result on strong normalization for $T$. Additionally, a slow growing analysis of Gödel's $T$ is obtained via Girard's hierarchy comparison theorem. This analyis yields a contribution to two open pro blems posed by Girard in part two of his book on proof theory. For the sake of completeness we also mention the Howard Schütte bound on derivation lengths for the simple typed $\lambda$-calculus.