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regular prime (Definition)

A prime $ p$ is regular if the class number of the cyclotomic field $ \mathbb{Q}(\zeta_p)$ is not divisible by $ p$ (where $ \zeta_p := e^{2 \pi i/p}$ denotes a primitive $ p^\mathrm{th}$ root of unity). An irregular prime is a prime that is not regular.

Regular primes rose to prominence as a result of Ernst Kummer's work in the 1850's on Fermat's Last Theorem. Kummer was able to prove Fermat's Last Theorem in the case where the exponent is a regular prime, a result that prior to Wiles's recent work was the only demonstration of Fermat's Last Theorem for a large class of exponents. In the course of this work Kummer also established the following numerical criterion for determining whether a prime is regular:

Based on this criterion it is possible to give a heuristic argument that the regular primes have density $ e^{-1/2}$ in the set of all primes [1]. Despite this, there is no known proof that the set of regular primes is infinite, although it is known that there are infinitely many irregular primes.

Bibliography

1
Kenneth Ireland & Michael Rosen, A Classical Introduction to Modern Number Theory, Springer-Verlag, New York, Second Edition, 1990.



"regular prime" is owned by djao.
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Also defines:  irregular prime

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examples of regular primes (Example) by alozano
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Cross-references: infinite, proof, density, argument, multiple, Bernoulli numbers, numerators, class, exponent, Fermat's last theorem, root of unity, primitive, divisible, cyclotomic field, class number, regular, prime
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This is version 3 of regular prime, born on 2002-06-05, modified 2005-04-20.
Object id is 3040, canonical name is RegularPrime.
Accessed 4670 times total.

Classification:
AMS MSC11R18 (Number theory :: Algebraic number theory: global fields :: Cyclotomic extensions)
 11R29 (Number theory :: Algebraic number theory: global fields :: Class numbers, class groups, discriminants)

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