# Vorlesungen über Zahlentheorie

*Vorlesungen über Zahlentheorie*(

*Lectures on Number Theory*) is a textbook of number theory written by German mathematicians P.G.L. Dirichlet and Richard Dedekind, and published in 1863.

Based on Dirichlet's number theory course at the University of Göttingen, the *Vorlesungen* were edited by Dedekind and published after Dirichlet's death. Dedekind added several appendices to the *Vorlesungen*, in which he collected further results of Dirichlet's and also developed his own original mathematical ideas.

Table of contents |

2 Contents 3 Importance 4 References |

## Scope

The*Vorlesungen*cover topics in elementary number theory, algebraic number theory and analytic number theory, including modular arithmetic, quadratic congruences, quadratic reciprocity and binary quadratic forms.

## Contents

The contents of Professor John Stillwell's 1999 translation of the*Vorlesungen*are as follows

- Chapter 1. On the divisibility of numbers
- Chapter 2. On the congruence of numbers
- Chapter 3. On quadratic residues
- Chapter 4. On quadratic forms
- Chapter 5. Determination of the class number of binary quadratic forms
- Supplement I. Some theorems from Gauss's theory of circle division
- Supplement II. On the limiting value of an infinite series
- Supplement III. A geometric theorem
- Supplement IV. Genera of quadratic forms
- Supplement V. Power residues for composite moduli
- Supplement VI. Primes in arithmetic progressions
- Supplement VII. Some theorems from the theory of circle division
- Supplement VIII. On the Pell equation
- Supplement IX. Convergence and continuity of some infinite series

Chapters 1 to 4 cover similar ground to Gauss' *Disquisitiones Arithmeticae*, and Dedekind added footnotes which specifically cross-reference the relevant sections of the *Disquisitiones*. These chapters can be thought of as a summary of existing knowledge, although Dirichlet simplifies Gauss' presentation, and introduces his own proofs in some places.

Chapter 5 contains Dirichlet's derivation of the class number formula for real and imaginary quadratic fields. Although other mathematicians had conjectured similar formulae, Dirichlet gave the first rigorous proof.

Supplement VI contains Dirichlet's proof that an arithmetic progression of the form *a*+*nd* where *a* and *d* are coprime contains an infinite number of primes.

## Importance

The*Vorlesungen*can be seen as a watershed between the classical number theory of Fermat, Jacobi and Gauss, and the modern number theory of Dedekind, Riemann and Hilbert. Dirichlet does not explicitly recognise the concept of the group that is central to modern algebra, but many of his proofs show an implicit understanding of group theory

The *Vorlesungen* contains two key results in number theory which were first proved by Dirichlet. The first of these is the class number formulae for binary quadratic forms. The second is a proof that arithmetic progressions contains an infinite number of primes (known as Dirichlet's theorem); this proof introduces Dirichlet L-series. These results are important milestones in the development of analytic number theory.

## References

- P.G.L. Dirichlet, R. Dedkind tr. John Stillwell:
*Lectures on Number Theory*, American Mathematical Society, 1999 ISBN 0821820176