# Iwasawa theory

In number theory, **Iwasawa theory** is the study of objects of arithmetic interest over infinite towers of number fields. It began as a Galois module theory of ideal class groups, initiated by Kenkichi Iwasawa (1959) (岩澤 健吉), as part of the theory of cyclotomic fields. In the early 1970s, Barry Mazur considered generalizations of Iwasawa theory to abelian varieties. More recently (early 1990s), Ralph Greenberg has proposed an Iwasawa theory for motives.

## Formulation

Iwasawa worked with so-called -extensions: infinite extensions of a number field with Galois group isomorphic to the additive group of p-adic integers for some prime *p*. Every closed subgroup of is of the form so by Galois theory, a -extension is the same thing as a tower of fields

such that Iwasawa studied classical Galois modules over by asking questions about the structure of modules over

More generally, Iwasawa theory asks questions about the structure of Galois modules over extensions with Galois group a p-adic Lie group.

## Example

Let be a prime number and let be the field generated over by the th roots of unity. Iwasawa considered the following tower of number fields:

where is the field generated by adjoining to the *p*^{n+1}-st roots of unity and

The fact that implies, by infinite Galois theory, that In order to get an interesting Galois module, Iwasawa took the ideal class group of , and let be its *p*-torsion part. There are norm maps whenever , and this gives us the data of an inverse system. If we set

then it is not hard to see from the inverse limit construction that is a module over In fact, is a module over the Iwasawa algebra . This is a 2-dimensional, regular local ring, and this makes it possible to describe modules over it. From this description it is possible to recover information about the *p*-part of the class group of

The motivation here is that the *p*-torsion in the ideal class group of had already been identified by Kummer as the main obstruction to the direct proof of Fermat's Last Theorem.

## Connections with p-adic analysis

From this beginning in the 1950s, a substantial theory has been built up. A fundamental connection was noticed between the module theory, and the p-adic L-functions that were defined in the 1960s by Kubota and Leopoldt. The latter begin from the Bernoulli numbers, and use interpolation to define p-adic analogues of the Dirichlet L-functions. It became clear that the theory had prospects of moving ahead finally from Kummer's century-old results on regular primes.

Iwasawa formulated the main conjecture of Iwasawa theory as an assertion that two methods of defining p-adic L-functions (by module theory, by interpolation) should coincide, as far as that was well-defined. This was proved by Mazur & Wiles (1984) for and for all totally real number fields by Wiles (1990). These proofs were modeled upon Ken Ribet's proof of the converse to Herbrand's theorem (the so-called Herbrand–Ribet theorem).

Karl Rubin found a more elementary proof of the Mazur-Wiles theorem by using Kolyvagin's Euler systems, described in Lang (1990) and Washington (1997), and later proved other generalizations of the main conjecture for imaginary quadratic fields.

## Generalizations

The Galois group of the infinite tower, the starting field, and the sort of arithmetic module studied can all be varied. In each case, there is a *main conjecture* linking the tower to a *p*-adic L-function.

In 2002, Chris Skinner and Eric Urban claimed a proof of a *main conjecture* for GL(2). In 2010, they posted a preprint (Skinner & Urban 2010).

## References

- Coates, J.; Sujatha, R. (2006),
*Cyclotomic Fields and Zeta Values*, Springer Monographs in Mathematics, Springer-Verlag, ISBN 978-3-540-33068-4, Zbl 1100.11002 - Greenberg, Ralph (2001), "Iwasawa theory---past and present", in Miyake, Katsuya (ed.),
*Class field theory---its centenary and prospect (Tokyo, 1998)*, Adv. Stud. Pure Math.,**30**, Tokyo: Math. Soc. Japan, pp. 335–385, ISBN 978-4-931469-11-2, MR 1846466, Zbl 0998.11054 - Iwasawa, Kenkichi (1959), "On Γ-extensions of algebraic number fields",
*Bulletin of the American Mathematical Society*,**65**(4): 183–226, doi:10.1090/S0002-9904-1959-10317-7, ISSN 0002-9904, MR 0124316, Zbl 0089.02402 - Kato, Kazuya (2007), "Iwasawa theory and generalizations" (PDF), in Sanz-Solé, Marta; Soria, Javier; Varona, Juan Luis; et al. (eds.),
*International Congress of Mathematicians. Vol. I*, Eur. Math. Soc., Zürich, pp. 335–357, doi:10.4171/022-1/14, ISBN 978-3-03719-022-7, MR 2334196 - Lang, Serge (1990),
*Cyclotomic fields I and II*, Graduate Texts in Mathematics,**121**, With an appendix by Karl Rubin (Combined 2nd ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-387-96671-7, Zbl 0704.11038 - Mazur, Barry; Wiles, Andrew (1984), "Class fields of abelian extensions of
**Q**",*Inventiones Mathematicae*,**76**(2): 179–330, doi:10.1007/BF01388599, ISSN 0020-9910, MR 0742853, Zbl 0545.12005 - Neukirch, Jürgen; Schmidt, Alexander; Wingberg, Kay (2008),
*Cohomology of Number Fields*, Grundlehren der Mathematischen Wissenschaften,**323**(Second ed.), Berlin: Springer-Verlag, doi:10.1007/978-3-540-37889-1, ISBN 978-3-540-37888-4, MR 2392026, Zbl 1136.11001 - Rubin, Karl (1991), "The 'main conjectures' of Iwasawa theory for imaginary quadratic fields",
*Inventiones Mathematicae*,**103**(1): 25–68, doi:10.1007/BF01239508, ISSN 0020-9910, Zbl 0737.11030 - Skinner, Chris; Urban, Éric (2010),
*The Iwasawa main conjectures for GL*(PDF), p. 219_{2} - Washington, Lawrence C. (1997),
*Introduction to cyclotomic fields*, Graduate Texts in Mathematics,**83**(2nd ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-387-94762-4 - Andrew Wiles (1990), "The Iwasawa Conjecture for Totally Real Fields",
*Annals of Mathematics*,**131**(3): 493–540, doi:10.2307/1971468, JSTOR 1971468, Zbl 0719.11071.

## Further reading

- de Shalit, Ehud (1987),
*Iwasawa theory of elliptic curves with complex multiplication.*p*-adic*L*functions*, Perspectives in Mathematics,**3**, Boston etc.: Academic Press, ISBN 978-0-12-210255-4, Zbl 0674.12004

## External links

- Hazewinkel, Michiel, ed. (2001) [1994], "Iwasawa theory",
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4