This chapter introduces Anderson and Belnap’s natural deduction treatment of entailment and the idea that hypotheses in deductions should really be used in those deductions. The idea of real use motivates relevant logic and is a key idea in the chapters that follow. The chapter outlines the development of Fitch-style natural deduction systems and introduces the reader to them.

This chapter derives asymptotics determined by a critical point where the singular variety is locally smooth: the generic situation which arises most commonly in practice. Several explicit formulae for asymptotics are given.

This chapter concludes the book. It contains a survey of the state of analytic combinatorics in several variables, including problems on the boundary of our current knowledge.

The semantical framework for the positive view of this book is one in which entailment is understood primarily in terms of theory closure. This chapter outlines both the history of the notion, beginning with Alfred Tarski’s theory of closure operators, and the relationship between closure operators and the entailment connective. At the end of the chapter, it is shown how closure operators can be used to model a simple logic, Graham Priest’s logic N4.

From 1912, C. I. Lewis attempted to construct a logic of entailment. In doing so, he created his modal logics, S1–S5, of which his chosen logic of entailment was S2. Although his logics avoid the so-called paradoxes of material implication, they still fall prey to the problem of explosion (that every proposition follows from any contradiction) and the problem of implosion (that every tautology follows from every proposition). These problems, and the inadequate treatment of nested entailments, make Lewis’s logics of limited use as logics of entailment. The chapter also discusses the systems devised by Lewis’s students Everett Nelson and William Parry. Nelson’s connexive logic avoids many of the problems with Lewis’s system but is found to have severe difficulties of its own, and Parry’s analytic implication, although it introduces an interesting version of the notion of meaning containment, does not adequately avoid the problems with Lewis’s logics.

This chapter generalizes the ideas given in the previous chapter. It sets out the notion of a model based on a set of theories. One of these theories is the logic itself. It is, so to speak, the correct theory of theories. It correctly states the principles under which all the theories (including itself) are closed. But each theory has associated with it a closure operator. Some of these operators get the principles of theory closure quite wrong in the sense that they do not apply correctly to every theory in the model. The interaction between these closure operators can be altered in various ways, giving rise to different logical systems. The resulting formal semantics can be represented in the manner of Kit Fine’s “Models for Entailment”.

This chapter contains a variety of examples deriving asymptotics of generating functions taken from the research literature, illustrating the power of analytic combinatorics in several variables.

This appendix presents a collection of key results on Morse theory, intersection classes, and the computation of Leray residue forms, specialized to the most important local geometries treated in the book.