Ordinal notations and induction

Abstract

AbstractIn order to prove that the simplification process for arithmetic eventually reaches a simple proof, it is necessary to measure the complexity of proofs in a more sophisticated way than for the cut-elimination theorem. There, a pair of numbers suffices, and the proof proceeds by double induction on this measure. This chapter develops the system of ordinal notations up to ε0 which serve as this more sophisticated measure for proofs in arithmetic. Ordinal notations are presented as purely combinatorial system of symbols, so that from the outset there is no doubt about the constructive legitimacy of the associated principles of reasoning. The main properties of this notation system are presented, and it is shown that ordinal notations are well-ordered according to its associated less-than relation. The basics of the theory of set-theoretic ordinals is developed in the second half of the chapter, so that the reader can compare the infinitary, set-theoretic development of ordinals up to ε0 to the system of finitary ordinal notations. Finally, Paris-Kirby Hydra game and Goodstein sequences are presented as applications of induction up to ε0.