Solving Causal Stream Inclusions

Abstract: We study solutions to systems of stream inclusions of the form 'f in T(f)', where the nondeterministic transformer 'T' on omega-infinite streams is assumed to be causal in the sense that elements in output streams are determined by a finite prefix of inputs. We first establish a correspondence between logic-based causality and metric-based contraction. Based on this causality-contraction connection we then apply fixpoint principles to the spherically complete ultrametric space of streams to construct solutions of stream inclusions. The underlying fixpoint iterations induce fixpoint induction principles to reason about these solutions.In addition, the fixpoint approximation provides an anytime algorithm with which finite prefixes of solutions can be calculated. These developments are illustrated for some central concepts of system design.