Which Numbers Are Real?

Michael Henle is Professor of Mathematics and Computer Science at Oberlin College and has had two visiting appointments, at Howard University and the Massachusetts Institute of Technology, as well as two semesters teaching in London in Oberlin's own program. He is the author of two books: A Combinatorial Introduction to Topology (W. H. Freeman and Co., 1978, reissued by Dover Publications, 1994) and Modern Geometries: The Analytic Approach (Prentice-Hall, 1996). He is currently editor of The College Mathematics Journal.

This work is a delightfully concise treatment of number systems. The number systems constructed here include the real, complex, quaternion, hyperreal, and surreal. Although numerous papers and books have been published about each of these systems, this treatise provides an introduction to all of them. As it is a categorical axiom system that characterizes the reals, all other number systems are compared to the real numbers. Henle (Oberlin College) constructs the reals twice, using both Cantor's construction and Dedekind cuts. He uses each of these constructions of the reals to motivate the construction of alternative number systems. In particular, the construction of the hyperreals utilizes ideas from Cantor's construction of the reals, and Dedekind cuts provide the motivation in constructing the surreals. The chapter on the constructive reals provides the reader with the historical perspective necessary to appreciate alternative number systems. The author also presents the geometry and calculus of each of the number systems included in this text within the context of the appropriate system."" - J.T. Zerger, CHOICE

• Introduction; Part I. The Reals: 1. Axioms for the reals; 2. Construction of the reals; Part II. Multidimensional Numbers: 3. The complex numbers; 4. The quaternions; Part III. Alternative Lines: 5. The constructive reals; 6. The hyperreals; 7. The surreals; Bibliography; Index.