PhD defence

Abstract. The subject of this thesis is the proof theory of linear logic with the least and greatest fixed points. In the literature, several systems have been studied for this language viz. the wellfounded system that relies on Park's induction rule, and systems that implicitly characterise induction such as the circular system and the non-wellfounded system. This thesis contributes to the theory of these systems with the ultimate goal of exactly capturing the provability relation of these systems and application of these objects in programming languages supporting (co)inductive reasoning. This thesis contains three parts. In the first part, we recall the literature on linear logic and the main approaches to the proof theory of logics with fixed points. In the second part, we obtain truth semantics for the wellfounded system, devise new wellfounded infinitely branching systems, and compute the complexity of provability in circular and non-wellfounded systems. In the third part, we devise non-wellfounded proof-nets and study their dynamics..