G¨odel& Recursivity JACQUES DUPARC B^atimentInternef CH - 1015 Lausanne
[email protected] Jacques.Duparc@epfl.ch 2 Contents Introduction 7 I Recursivity 9 1 Towards Turing Machines 11 1.1 Deterministic Finite Automata . 11 1.2 Nondeterministic Finite Automata . 12 1.3 Regular Expressions . 15 1.4 Non-Regular Languages . 19 1.5 Pushdown Automata . 20 1.6 Context-Free Grammar . 22 2 Turing Machines 27 2.1 Deterministic Turing Machines . 27 2.2 Non-Deterministic Turing Machines . 32 2.3 The Concept of Algorithm . 35 2.4 Universal Turing Machine . 37 2.5 The Halting Problem . 40 2.6 Turing Machine with Oracle . 41 3 Recursive Functions 51 3.1 Primitive Recursive Functions . 51 3.2 Variable Substitution . 56 3.3 Bounded Minimisation and Bounded Quantification . 58 3.4 Coding Sequences of Integers . 60 3.5 Partial Recursive Functions . 62 II Arithmetic 75 4 Representing Functions 77 4.1 Robinson Arithmetic . 77 4.2 Representable Functions . 83 4 CONTENTS 5 G¨odelFirst Incompleteness Theorem 111 5.1 G¨odelNumbers . 111 5.2 Coding the Proofs . 123 5.3 Undecidability of Robinson Arithmetic . 138 Introduction Introduction 7 The basic requirements for this course are contained in the "Mathematical Logic" course. Among other things, you should have a clear understanding of each of the following: first order language, signature, terms, formulas, theory, proof theory, models, completeness theorem, compactness theorem, L¨owenheim-Skolem theorem. It makes no sense to take this course without this solid background on first order logic. The title of the course is ”G¨odeland Recursivity" but it should rather be "Recursivity and G¨odel"since that is the way we are going to go through these topics (1) Recursivity (2) G¨odel'sincompleteness theorems (there are two of them) Recursivity is at the heart of computer science, it represents the mathematical side of what computing is like.