By a result of Paris and Friedman, the collection axiom schema for $\Sigma_{n+1}$ formulas, $B\Sigma_{n+1}$ , is $\Pi_{n+2}$ conservative over $I\Sigma_n$ . We give a new proof-theoretic proof of this theorem, which is based on a reduction of $B\Sigma_n$ to a version of collection rule and a subsequent analysis of this rule via Herbrand's theorem. A generalization of this method allows us to improve known results on reflection principles for $B\Sigma_n$ and to answer some technical questions left open by Sieg (...) [23] and Hájek [9]. We also give a new proof of independence of $B\Sigma_{n+1}$ over $I\Sigma_n$ by a direct recursion-theoretic argument and answer an open problem formulated by Gaifman and Dimitracopoulos [8]. (shrink)

A well-known result states that, over basic Kalmar elementary arithmetic EA, the induction schema for ∑n formulas is equivalent to the uniform reflection principle for ∑n + 1 formulas . We show that fragments of arithmetic axiomatized by various forms of induction rules admit a precise axiomatization in terms of reflection principles as well. Thus, the closure of EA under the induction rule for ∑n formulas is equivalent to ω times iterated ∑n reflection principle. Moreover, for k < ω, k (...) times iterated ∑n reflection principle over EA precisely corresponds to the extension of EA by k nested applications of ∑n induction rule.The above relationship holds in greater generality than just stated. In fact, we give general formulas characterizing in terms of iterated reflection principles the extension of any given theory by k nested applications of ∑n or Πn induction rules. In particular, the closure of a theory T under just one application of ∑1 induction rule is equivalent to T together with ∑1 reflection principle for each finite Π2 axiomatized subtheory of T.These results have closely parallel ones in the theory of subrecursive function classes. The rules under study correspond, in a canonical way, to natural closure operators on the classes of provably recursive functions. Thus, ∑1 induction rule precisely corresponds to the primitive recursive closure operator, and ∑1 collection rule, introduced below, corresponds to the elementary closure operator. (shrink)

Volume II of Classical Recursion Theory describes the universe from a local (bottom-up or synthetical) point of view, and covers the whole spectrum, from the recursive to the arithmetical sets. The first half of the book provides a detailed picture of the computable sets from the perspective of Theoretical Computer Science. Besides giving a detailed description of the theories of abstract Complexity Theory and of Inductive Inference, it contributes a uniform picture of the most basic complexity classes, ranging from small (...) time and space bounds to the elementary functions, with a particular attention to polynomial time and space computability. It also deals with primitive recursive functions and larger classes, which are of interest to the proof theorist. The second half of the book starts with the classical theory of recursively enumerable sets and degrees, which constitutes the core of Recursion or Computability Theory. Unlike other texts, usually confined to the Turing degrees, the book covers a variety of other strong reducibilities, studying both their individual structures and their mutual relationships. The last chapters extend the theory to limit sets and arithmetical sets. The volume ends with the first textbook treatment of the enumeration degrees, which admit a number of applications from algebra to the Lambda Calculus. The book is a valuable source of information for anyone interested in Complexity and Computability Theory. The student will appreciate the detailed but informal account of a wide variety of basic topics, while the specialist will find a wealth of material sketched in exercises and asides. A massive bibliography of more than a thousand titles completes the treatment on the historical side. (shrink)

Herbrand's Theorem, in the form of $$\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{\exists } $$ -inversion lemmata for finitary and infinitary sequent calculi, is the crucial tool for the determination of the provably total function(al)s of a variety of theories. The theories are (second order extensions of) fragments of classical arithmetic; the classes of provably total functions include the elements of the Polynomial Hierarchy, the Grzegorczyk Hierarchy, and the extended Grzegorczyk Hierarchy $\mathfrak{E}^\alpha $ , α < ε0. A subsidiary aim of the paper is to show (...) that the proof theoretic methods used here are distinguished by technical elegance, conceptual clarity, and wide-ranging applicability. (shrink)

It is well known that the structure of honest elementary degrees is a lattice with rather strong density properties. Let $\mbox{\bf a} \cup \mbox{\bf b}$ and $\mbox{\bf a} \cap \mbox{\bf b}$ denote respectively the join and the meet of the degrees $\mbox{\bf a}$ and $\mbox{\bf b}$ . This paper introduces a jump operator ( $\cdot'$ ) on the honest elementary degrees and defines canonical degrees $\mbox{\bf 0},\mbox{\bf 0}', \mbox{\bf 0}^{\prime \prime },\ldots$ and low and high degrees analogous to the corresponding (...) concepts for the Turing degrees. Among others, the following results about the structure of the honest elementary degrees are shown: There exist low degrees, and there exist degrees which are neither low nor high. Every degree above $\mbox{\bf 0}'$ is the jump of some degree, moreover, for every degree $\mbox{\bf c}$ above $\mbox{\bf 0}'$ there exist degrees $\mbox{\bf a},\mbox{\bf b}$ such that $\mbox{\bf c}=\mbox{\bf a} \cup \mbox{\bf b} = \mbox{{\bf a}}'=\mbox{\bf b}'$ . We have $\mbox{\bf a}'\cup \mbox{\bf b}' \leq (\mbox{\bf a}\cup\mbox{\bf b})'$ and $\mbox{\bf a}'\cap \mbox{\bf b}' \geq (\mbox{\bf a}\cap \mbox{\bf b})'$ . The jump operator is of course monotonic, i.e. $\mbox{\bf a}\leq\mbox{{\bf b}}\Rightarrow \mbox{\bf a}'\leq \mbox{\bf b}'$ . We prove that every situation compatible with $\mbox{\bf a}\leq\mbox{\bf b}\Rightarrow \mbox{\bf a}'\leq \mbox{\bf b}'$ is realized in the structure, e.g. we have incomparable degrees $\mbox{\bf a},\mbox{\bf b}$ such that $\mbox{\bf a}'<\mbox{\bf b}'$ and incomparable degrees $\mbox{\bf a},\mbox{\bf b}$ such that $\mbox{\bf a}' = \mbox{\bf b}'$ etcetera. We are able to prove all these results without the traditional recursion theoretic constructions. Our proof method relies on the fact that the growth of the functions in a degree is bounded. This technique also yields a very simple proof of an old result, namely that the structure is a lattice. (shrink)