In mathematics, an Euler system is a collection of compatible elements of Galois cohomology groups indexed by fields. They were introduced by in his work on Heegner points on modular elliptic curves, which was motivated by his earlier paper and the work of. Euler systems are named after Leonhard Euler because the factors relating different elements of an Euler system resemble the Euler factors of an Euler product. Euler systems can be used to construct annihilators of ideal class groups or Selmer groups, thus giving bounds on their orders, which in turn has led to deep theorems such as the finiteness of some Tate-Shafarevich groups. This led to Karl Rubin's new proof of the main conjecture of Iwasawa theory, considered simpler than the original proof due to Barry Mazur and Andrew Wiles.
Definition
Although there are several definitions of special sorts of Euler system, there seems to be no published definition of an Euler system that covers all known cases. But it is possible to say roughly what an Euler system is, as follows:
An Euler system is given by collection of elements cF. These elements are often indexed by certain number fieldsF containing some fixed number fieldK, or by something closely related such as square-free integers. The elements cF are typically elements of some Galois cohomology group such as H1 where T is a p-adic representation of the absolute Galois group of K.
The most important condition is that the elementscF and cG for two different fields F ⊆ G are related by a simple formula, such as
There may be other conditions that the cF have to satisfy, such as congruence conditions.
Kazuya Kato refers to the elements in an Euler system as "arithmetic incarnations of zeta" and describes the property of being an Euler system as "an arithmetic reflection of the fact that these incarnations are related to special values of Euler products".
For every square-free positive integern pick an n-th root ζn of 1, with ζmn = ζmζn for m,n coprime. Then the cyclotomic Euler system is the set of numbers αn = 1 − ζn. These satisfy the relations where l is a prime not dividing n and Fl is a Frobenius automorphism with Fl = ζ. Kolyvagin used this Euler system to give an elementary proof of the Gras conjecture. Pen
Kolyvagin constructed an Euler system from the Heegner points of an elliptic curve, and used this to show that in some cases the Tate-Shafarevich group is finite.
