Egbert Rijke’s Elementary Number Theory is a substantial set of upper-undergraduate lecture notes designed not merely to teach standard number theory, but also to develop proof-writing, structural thinking, and mathematical problem-solving. The notes are associated with Johns Hopkins University’s Math 304 course and are explicitly designed with minimal prerequisites; the 2026 version contains 479 pages and 284 exercises.
The opening chapters establish mathematical foundations through induction, recursion, bijections, counting, the well-ordering principle, and Euclidean division. This is followed by divisibility, greatest common divisors, Euclid’s algorithm, linear Diophantine equations, congruences, and the construction and interpretation of the integers and rationals.
The central theme then becomes the arithmetic structure of integers. Students encounter prime numbers through the Fundamental Theorem of Arithmetic, proofs of the infinitude of primes, Fermat primes, Bertrand’s postulate, and arithmetic functions such as Euler’s totient and the Möbius function. Modular arithmetic becomes a major organizing tool, leading to the Chinese Remainder Theorem, multiplicative orders, Fermat’s Little Theorem, Euler’s theorem, Wilson’s theorem, and Lucas’s theorem. The notes also explore Pythagorean triples, continued fractions, Vieta jumping, infinite descent, and the congruent-number problem, illustrating how apparently elementary equations can lead to sophisticated proof techniques.
A particularly strong feature is the transition from elementary integer arithmetic to more algebraic structures. The notes introduce Gaussian integers $\mathbb{Z}[i]$, polynomial arithmetic, primitive roots, polynomial congruences, Hensel’s lemma, irreducibility, Eisenstein’s criterion, cyclotomic polynomials, and Zsigmondy’s theorem. Fermat’s Two-Squares Theorem is approached through several different methods, including Euler’s infinite descent and a proof based on fixed points of involutions. This multiple-proof philosophy is important pedagogically: rather than presenting theorems as isolated facts, Rijke uses them to demonstrate different ways mathematical ideas can arise and interact.
The final major section develops quadratic residues and quadratic reciprocity, culminating in Gauss’s lemma and Eisenstein’s proof. The notes therefore progress from very concrete questions about divisibility and integer equations toward increasingly abstract ideas involving algebraic structures, polynomial factorization, and quadratic forms. Historical development is deliberately integrated throughout: the material discusses mathematicians including Fermat, Euler, Gauss, Fibonacci, and ancient Greek and Indian mathematicians. Rijke explicitly emphasizes asking how mathematicians historically thought about problems and why particular structures emerge, making the notes as much a course in mathematical reasoning as a catalogue of number-theoretic results.
Key takeaways
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The opening chapters establish mathematical foundations through induction, recursion, bijections, counting, the well-ordering principle, and Euclidean division. This is followed by divisibility, greatest common divisors, Euclid’s algorithm, linear Diophantine equations, congruences, and the construction and interpretation of the integers and rationals.
The central theme then becomes the arithmetic structure of integers. Students encounter prime numbers through the Fundamental Theorem of Arithmetic, proofs of the infinitude of primes, Fermat primes, Bertrand’s postulate, and arithmetic functions such as Euler’s totient and the Möbius function. Modular arithmetic becomes a major organizing tool, leading to the Chinese Remainder Theorem, multiplicative orders, Fermat’s Little Theorem, Euler’s theorem, Wilson’s theorem, and Lucas’s theorem. The notes also explore Pythagorean triples, continued fractions, Vieta jumping, infinite descent, and the congruent-number problem, illustrating how apparently elementary equations can lead to sophisticated proof techniques.
A particularly strong feature is the transition from elementary integer arithmetic to more algebraic structures. The notes introduce Gaussian integers $\mathbb{Z}[i]$, polynomial arithmetic, primitive roots, polynomial congruences, Hensel’s lemma, irreducibility, Eisenstein’s criterion, cyclotomic polynomials, and Zsigmondy’s theorem. Fermat’s Two-Squares Theorem is approached through several different methods, including Euler’s infinite descent and a proof based on fixed points of involutions. This multiple-proof philosophy is important pedagogically: rather than presenting theorems as isolated facts, Rijke uses them to demonstrate different ways mathematical ideas can arise and interact.
The final major section develops quadratic residues and quadratic reciprocity, culminating in Gauss’s lemma and Eisenstein’s proof. The notes therefore progress from very concrete questions about divisibility and integer equations toward increasingly abstract ideas involving algebraic structures, polynomial factorization, and quadratic forms. Historical development is deliberately integrated throughout: the material discusses mathematicians including Fermat, Euler, Gauss, Fibonacci, and ancient Greek and Indian mathematicians. Rijke explicitly emphasizes asking how mathematicians historically thought about problems and why particular structures emerge, making the notes as much a course in mathematical reasoning as a catalogue of number-theoretic results.
Key takeaways
- Proof comes first: induction, well-ordering, bijections, and proof-writing provide the foundation for later number theory.
- Euclidean division is fundamental: it leads naturally to divisibility, gcds, Euclid’s algorithm, and Diophantine equations.
- Prime factorization organizes arithmetic: every integer greater than $1$ has a unique factorization into primes.
- Congruences turn arithmetic into a structural system: $a\equiv b\pmod n$ becomes the basis for modular arithmetic.
- The Chinese Remainder Theorem provides a systematic way to combine simultaneous congruences.
- Fermat, Euler, and Wilson: their theorems reveal deep relationships between primes and modular arithmetic.
- Pythagorean triples connect Diophantine equations, geometry, rational points, and parametrization.
- Infinite descent is a powerful method for proving that certain integer solutions cannot exist.
- Primitive roots and discrete logarithms introduce important ideas that later appear in algebra and cryptography.
- Gaussian integers demonstrate how extending $\mathbb Z$ to $\mathbb Z[i]$ can make difficult integer problems easier.
- Quadratic residues study when equations such as $x^2\equiv a\pmod p$ have solutions.
- Quadratic reciprocity reveals a remarkable symmetry between different primes.
- Multiple proofs matter: the notes repeatedly show that the same theorem can illuminate different mathematical ideas depending on how it is proved.
- Historical context is part of the mathematics: the development from Fermat to Euler to Gauss is used to show how mathematical structures and methods evolved.
- For teaching: the material is particularly valuable for connecting secondary-school topics—divisibility, primes, modular arithmetic, Diophantine equations, induction, and Pythagorean triples—to genuine university-level proof mathematics.
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