In mathematics, an Euler system is a collection of compatible elements of Galois cohomology groups indexed by fields. They were introduced by Kolyvagin (1990) in his work on Heegner points on modular elliptic curves, which was motivated by his earlier paper Kolyvagin (1988) and the work of Thaine (1988). 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.
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 fields F containing some fixed number field K, or by something closely related such as square-free integers. The elements cF are typically elements of some Galois cohomology group such as H1(F, T) where T is a p-adic representation of the absolute Galois group of K.
- The most important condition is that the elements cF and cG for two different fields F ⊆ G are related by a simple formula, such as
- Here the "Euler factor" P(τ|B;x) is defined to be the element det(1-τx|B) considered as an element of O[x], which when x happens to act on B is not the same as det(1-τx|B) considered as an element of O.
- 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".
Cyclotomic units 
For every square-free positive integer n 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
- modulo all primes above l
where l is a prime not dividing n and Fl is a Frobenius automorphism with Fl(ζn) = ζl
n. Kolyvagin used this Euler system to give an elementary proof of the Gras conjecture.
Gauss sums 
Elliptic units 
Heegner points 
Kato's Euler system 
Kato's Euler system consists of certain elements occurring in the algebraic K-theory of modular curves. These elements—named Beilinson elements after Alexander Beilinson who introduced them in Beilinson (1984)—were used by Kazuya Kato in Kato (2004) to prove one divisibility in Barry Mazur's main conjecture of Iwasawa theory for elliptic curves.
- Banaszak, Grzegorz (2001), "Euler systems for number fields", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- Beilinson, Alexander (1984), "Higher regulators and values of L-functions", in R. V. Gamkrelidze, Current problems in mathematics (in Russian) 24, pp. 181–238, MR 0760999
- Coates, J.H.; Greenberg, R.; Ribet, K.A.; Rubin, K. (1999), Arithmetic Theory of Elliptic Curves, Lecture Notes in Mathematics 1716, Springer-Verlag, ISBN 3-540-66546-3
- Coates, J.; Sujatha, R. (2006), "Euler systems", Cyclotomic Fields and Zeta Values, Springer Monographs in Mathematics, Springer-Verlag, pp. 71–87, ISBN 3-540-33068-2
- Kato, Kazuya (2004), "p-adic Hodge theory and values of zeta functions of modular forms", in Pierre Berthelot, Jean-Marc Fontaine, Luc Illusie, Kazuya Kato, and Michael Rapoport, Cohomologies p-adiques et applications arithmétiques. III., Astérisque 295, Paris: Société Mathématique de France, pp. 117–290, MR 2104361
- Kato, Kazuya (2007), "Iwasawa theory and generalizations", in Marta Sanz-Solé, Javier Soria, Juan Luis Varona, and Joan Verdera, International Congress of Mathematicians I, Zürich: European Mathematical Society, pp. 335–357, MR 2334196, retrieved 2010-08-12. Proceedings of the congress held in Madrid, August 22–30, 2006
- Kolyvagin, V. A. (1988), "The Mordell-Weil and Shafarevich-Tate groups for Weil elliptic curves", Izvestiya Akademii Nauk SSSR. Seriya Matematicheskaya 52 (6): 1154–1180, ISSN 0373-2436, MR 984214
- Kolyvagin, V. A. (1990), "Euler systems", The Grothendieck Festschrift, Vol. II, Progr. Math. 87, Boston, MA: Birkhäuser Boston, pp. 435–483, doi:10.1007/978-0-8176-4575-5_11, ISBN 978-0-8176-3428-5, MR 1106906
- Mazur, Barry; Rubin, Karl (2004), "Kolyvagin systems", Memoirs of the American Mathematical Society 168 (799): viii+96, ISBN 978-0-8218-3512-8, ISSN 0065-9266, MR 2031496
- Rubin, Karl (2000), Euler systems, Annals of Mathematics Studies 147, Princeton University Press, MR 1749177
- Scholl, A. J. (1998), "An introduction to Kato's Euler systems", Galois representations in arithmetic algebraic geometry (Durham, 1996), London Math. Soc. Lecture Note Ser. 254, Cambridge University Press, pp. 379–460, ISBN 978-0-521-64419-8, MR 1696501
- Thaine, Francisco (1988), "On the ideal class groups of real abelian number fields", Annals of Mathematics. Second Series 128 (1): 1–18, doi:10.2307/1971460, ISSN 0003-486X, MR 951505
- Several papers on Kolyvagin systems are available at Barry Mazur's web page (as of July 2005).