Introducing Second-Order Spider Diagrams for Defining Regular Languages

There has been significant research effort focussed on the study of regular languages, since they play a vital role in our understanding of computation. This existing research draws a large number of connections with other areas, such as algebra and symbolic logic. Recently, research has begun into how diagrammatic logics can define regular languages, providing another mechanism through which we can understand regular languages. However, the formalised diagrammatic logics are first-order, so they cannot define non-starfree regular languages. The primary contributions of this paper are: (a) to develop and formalise a second-order diagrammatic logic, extending spider diagrams of order, and (b) to establish a class of regular languages that this logic can define. This lays the essential foundations for providing an exact classification of the regular languages that are definable using this new second-order logic.