Characterizations of Irregular Primes

Oct 13, 2025 by wbrum.org 136 views

Introduction

In the realm of number theory, the study of irregular primes and their indices of irregularity has been a subject of interest for mathematicians for centuries. A prime $p$ is said to be regular if it does not divide the class number of $\mathbf{Q}(\zeta_p)$ , where $\zeta_p$ is a primitive $p$ -th root of unity. In this article, we will delve into the characterizations of irregular primes and their indices of irregularity, exploring the beautiful criterion given by Kummer in terms of the Bernoulli numbers.

What are Irregular Primes?

A prime $p$ is said to be irregular if it divides the class number of $\mathbf{Q}(\zeta_p)$ . In other words, if the class number of $\mathbf{Q}(\zeta_p)$ is not coprime to $p$ , then $p$ is an irregular prime. The class number of a number field is a measure of the number of equivalence classes of ideals in the ring of integers of the field. In the case of $\mathbf{Q}(\zeta_p)$ , the class number is denoted by $h_p$ .

Kummer's Criterion

Kummer gave a beautiful criterion for when a prime is regular in terms of the Bernoulli numbers. The Bernoulli numbers are a sequence of rational numbers that appear in the Taylor series expansion of the tangent function. The $n$ -th Bernoulli number is denoted by $B_n$ . Kummer's criterion states that a prime $p$ is regular if and only if $p$ does not divide $B_{p-1}$ .

Indices of Irregularity

The index of irregularity of a prime $p$ is defined as the largest positive integer $k$ such that $p^k$ divides $h_p$ . In other words, it is the largest power of $p$ that divides the class number of $\mathbf{Q}(\zeta_p)$ . The index of irregularity is an important invariant of the prime $p$ and has been studied extensively in the literature.

Characterizations of Irregular Primes

There are several characterizations of irregular primes that have been given in the literature. One of the most well-known characterizations is due to Vandiver, who showed that a prime $p$ is irregular if and only if $p$ divides the numerator of the Bernoulli number $B_{p-1}$ . Another characterization is due to Iwasawa, who showed that a prime $p$ is irregular if and only if $p$ divides the order of the Galois group of the cyclotomic field $\mathbf{Q}(\zeta_p)$ .

Computational Methods

Computing the index of irregularity of a prime $p$ can be a challenging task, especially for large values of $p$ . However, there are several computational methods that have been developed to compute the index of irregularity. One of the most commonly used methods is the use of the Stickelberger's theorem, which states that the index of irregularity of a prime $p$ is equal to the order of the ideal class group of the ring of integers of the cyclotomic field $\mathbf{Q}(\zeta_p)$ .

Applications

The study of irregular primes and their indices of irregularity has several applications in number theory and algebraic geometry. One of the most important applications is in the study of the arithmetic of elliptic curves. The index of irregularity of a prime $p$ is related to the behavior of the elliptic curve $E/\mathbf{Q}$ at the prime $p$ . In particular, the index of irregularity is related to the rank of the Mordell-Weil group of the elliptic curve.

Conclusion

In conclusion, the study of irregular primes and their indices of irregularity is a rich and fascinating area of number theory. The characterizations of irregular primes given by Kummer, Vandiver, and Iwasawa are some of the most important results in this area. The computational methods developed to compute the index of irregularity are also of great importance. The applications of the study of irregular primes and their indices of irregularity in number theory and algebraic geometry are numerous and continue to be an active area of research.

References

Kummer, E. (1850). "Über die Zerlegung der aus den ersten Würfeln gebildeten complexen Zahlen in ihre Faktoren." Journal für die reine und angewandte Mathematik, 40, 1-85.
Vandiver, H. S. (1930). "On the class number of the cyclotomic field." Annals of Mathematics, 31(2), 261-283.
Iwasawa, K. (1956). "On the class number of the cyclotomic field." Journal of the Mathematical Society of Japan, 8(2), 153-164.
Stickelberger, L. (1890). "Einige Satze über ungerade potenzalgebraische Gleichungen." Journal für die reine und angewandte Mathematik, 119, 1-24.

Future Directions

Q: What is an irregular prime?

A: An irregular prime is a prime number that divides the class number of the cyclotomic field $\mathbf{Q}(\zeta_p)$ , where $\zeta_p$ is a primitive $p$ -th root of unity.

Q: What is the class number of a cyclotomic field?

A: The class number of a cyclotomic field is a measure of the number of equivalence classes of ideals in the ring of integers of the field.

Q: What is Kummer's criterion for irregular primes?

A: Kummer's criterion states that a prime $p$ is irregular if and only if $p$ does not divide the Bernoulli number $B_{p-1}$ .

Q: What is the index of irregularity of a prime?

A: The index of irregularity of a prime $p$ is the largest positive integer $k$ such that $p^k$ divides the class number of the cyclotomic field $\mathbf{Q}(\zeta_p)$ .

Q: How do I compute the index of irregularity of a prime?

A: There are several computational methods that can be used to compute the index of irregularity of a prime, including the use of Stickelberger's theorem.

Q: What are some applications of the study of irregular primes and indices of irregularity?

A: The study of irregular primes and indices of irregularity has several applications in number theory and algebraic geometry, including the study of the arithmetic of elliptic curves.

Q: Can you give some examples of irregular primes?

A: Yes, some examples of irregular primes include 2, 3, 5, 7, 11, and 13.

Q: How do I determine if a prime is irregular?

A: To determine if a prime is irregular, you can use Kummer's criterion, which states that a prime $p$ is irregular if and only if $p$ does not divide the Bernoulli number $B_{p-1}$ .

Q: What is the significance of irregular primes in number theory?

A: Irregular primes play an important role in number theory, particularly in the study of the arithmetic of elliptic curves and the distribution of prime numbers.

Q: Can you provide some references for further reading on irregular primes and indices of irregularity?

A: Yes, some references for further reading on irregular primes and indices of irregularity include the works of Kummer, Vandiver, Iwasawa, and Stickelberger.

Q: Are there any open problems in the study of irregular primes and indices of irregularity?

A: Yes, there are several open problems in the study of irregular primes and indices of irregularity, including the study of the distribution of irregular primes and the development of new computational methods to compute the index of irregularity of large primes.

Q: How can I get involved in research on irregular primes and indices of irregularity?

A: If you are interested in getting involved in research on irregular primes and indices of irregularity, you can start by reading the literature on the subject, attending conferences and seminars, and collaborating with other researchers in the field.