Anthony Kay was a Lecturer in Mathematical Sciences at Loughborough University for 32 years up to his retirement in 2020. Although his research has been in applications of mathematics, he has taught a wide range of topics in pure and applied mathematics to students at all levels, from first year to postgraduate.

1. Introduction: The Purpose of this Book. 1.1. A Very Brief Historical Context. 1.2. The Axiomatic Method. 1.3. The Place of Number Systems within Mathematics. 1.4. Mathematical Writing, Notation and Terminology. 1.5. Logic and Methods of Proof. 2. Sets and Relations. 2.1. Sets. 2.2. Relations between Sets. 2.3. Relations on a Set. 3. Natural Number, N. 3.1. Peano's Axioms. 3.2. Addition of Natural Numbers. 3.3. Multiplication of Natural Numbers. 3.4. Exponentiation (Powers) of Natural Numbers. 3.5. Order in the Natural Numbers. 3.6. Bounded Sets in N. 3.7. Cardinality, Finite and Infinite Sets. 3.8. Subtraction: the Inverse of Addition. 4. Integers, Z. 4.1. Definition of the Integers. 4.2. Arithmetic on Z. 4.3. Algebraic Structure of Z. 4.4. Order in Z. 4.5 Finite, Infinite and Bounded Sets in Z. 5. Foundations of Number Theory. 5.1. Integer Division. 5.2. Expressing Integers in any Base. 5.3. Prime Numbers and Prime Factorisation. 5.4. Congruence. 5.5. Modular Arithmetic. 5.6. Zd as an Algebraic Structure. 6. Rational Numbers, Q. 6.1 Definition of the Rationals. 6.2. Addition and Multiplication on Q. 6.3. Countability of Q. 6.4. Exponentiation and its Inverse(s) on Q. 6.5. Order in Q. 6.6. Bounded Sets in Q. 6.7. Expressing Rational Numbers in any Base. 6.8. Sequences and Series. 7. Real Numbers, R. 7.1. The Requirements for our Next Number System. 7.2. Dedekind Cuts. 7.3. Order and Bounded Sets in R. 7.4 Addition in R. 7.5. Multiplication in R. 7.6. Exponentiation in R. 7.7. Expressing Real Numbers in any Base. 7.8. Cardinality of R. 7.9. Algebraic and Transcendental Numbers. 8. Quadratic Extensions I: General Concepts and Extensions of Z and Q. 8.1. General Concepts of Quadratic Extensions. 8.2. Introduction to Quadratic Rings: Extensions of Z. 8.3. Units in Z[¿k]. 8.4. Primes in Z[¿k]. 8.5. Prime Factorisation in Z[¿k. 8.6. Quadratic Fields: Extensions of Q. 8.7. Norm-Euclidean Rings and Unique Prime Factorisation. 9. Quadratic Extensions II: Complex Numbers, C. 9.1. Complex Numbers as a Quadratic Extension. 9.2. Exponentiation by Real Powers in C: a First Approach. Geometry of C; the Principal Value of the Argument, and the Number ¿. 9.4. Use of the Argument to Define Real Powers in C. 9.5. Exponentiation by Complex Powers; the Number e. 9.6. The Fundamental Theorem of Algebra. 9.7. Cardinality of C. 10. Yet More Number Systems. 10.1. Constructible Numbers. 10.2. Hypercomplex Numbers. 11. Where Do We Go From Here? 11.1. Number Theory and Abstract Algebra. 11.2. Analysis. A. How to Read Proofs: The `Self-Explanation' Strategy.