Overview of Some Iwasawa Theory

Overview of some Iwasawa theory Christian Wuthrich 23rd | 27th of July 2012 Contents 0 Introduction 1 1 Iwasawa theory of the class group 1 2 Iwasawa theory for elliptic curves 8 3 The leading term formula 14 4 Selmer groups for general Galois representations 21 5 Kato's Euler system 22 0 Introduction This is the draft version of the notes to my lectures at Heidelberg in July 2012. The intention is to give an overview of some topics in Iwasawa theory. These lectures will contain a lot of definitions and results, but hardly any proofs and details. Especially I would like to emphasise that the word \proof" should actually be replaced by \sketch of proof" in all cases below. Also I have no claim at making this a complete introduction to the subject, nor is the list of references at the end. For this the reader might find [14] a better source. All computations were done in [46]. Any comments and corrections are very welcome. It is my pleasure to thank Thanasis Bouganis, Sylvia Guibert, Chern-Yang Lee, Birgit Schmoetten-Jonas and Otmar Venjakob. 1 Iwasawa theory of the class group Let F be a number field and let p be an odd prime. Suppose we are given a tower of Galois extensions F = 0F ⊂ 1F ⊂ 2F ⊂ · · · such that the Galois group of nF=F is cyclic n n n of order p for all n > 1. Write C for the p-primary part of the class group of F and write pen for its order. Theorem 1 (Iwasawa 56 [15]). There exist integers µ, λ, ν, and n0 such that n en = µ p + λ n + ν for all n > n0. 1 Iwasawa theory cw 12 1.1 Zp-extensions Let me first describe the tower of extensions that we are talking about. Set 1F = S nF . 1 The extension F=F is called a Zp-extension as its Galois group Γ is isomorphic to the additive group of p-adic integers since it is the projective limit of cyclic groups of order n p . The most important example is the cyclotomic Zp-extension: If F = Q, then the × n−1 n Z n Galois group of Q(µp )=Q is =p Z , which is cyclic of order (p − 1)p . So there is n−1 1 an extension Q=Q contained in Q(µpn ) such that Q is a Zp-extension of Q. For a 1 1 general F , the cyclotomic Zp-extension is F = Q · F . 1 It follows from the Kronecker-Weber theorem that Q is the unique Zp-extension of Q. It would be a consequence of Leopoldt's conjecture that the cyclotomic Zp-extension is the only one for any totally real number field, see [29, Theorem 11.1.2]. For a general r2+1 number field F , the composition of all Zp-extension contains at least Zp in its Galois group where r2 denotes the number of complex places in F . For a imaginary quadratic number field F , for instance, the theory of elliptic curves with complex multiplication provides us with another interesting Zp-extension, the anti-cyclotomic Zp-extension. It 1 1 can be characterised as the only Zp-extension F=F such that F=Q is a non-abelian Galois extension. Lemma 2 (Proposition 11.1.1 in [29]). The only places that can ramify in 1F=F divide p and at least one of them must ramify. In the cyclotomic Zp-extension of F , all places above p are ramified and there are only finitely many places above all other places. 1.2 The Iwasawa algebra and its modules Let O be a \coefficient ring", for us this will always be the ring of integers in a p-adic field; n so O = Zp is typical. There is a natural morphism between the group rings O Gal( F=F ) , which allows us to form the limit Λ = lim OGal(nF=F ): − n This completed topological group ring, called the Iwasawa algebra and also denoted by O[[Γ]], is far better to work with than the huge group ring O[Γ]. Proposition 3. To a choice of a topological generator γ of Γ, there is an isomorphism from Λ to the ring of formal power series O[[T ]] sending γ to T + 1. The proof is given in Theorem 5.3.5 of [29]. By the Weierstrass preparation theorem, an element f(T ) 2 O[[T ]] can be written as a product of a power of the uniformiser of O times a unit of O[[T ]] and times a distinguished polynomial, which, by definition, is a monic polynomial whose non-leading coefficients belong to the maximal ideal. Let nX be a system of abelian groups with an action by Gal(nF=F ). If there is a naturally defined norm map n+1X ! nX, then we can form X = lim nX and consider it − as a compact Λ-module. For instance the class groups nC above have a natural norm map between them. Also lots of naturally defined cohomology groups will have such a map, too. Suppose now O=Zp is unramified, otherwise the power of p below must be replaced by a power of the uniformiser of O. 2 Iwasawa theory cw 12 Proposition 4. Let X be a finitely generated Λ-module. Then there exist integers r, s, t, m1, m2,..., ms, n1, n2,... nt, irreducible distinguished polynomials f1, f2,... ft, and a morphism of Λ-modules s t r M Λ M Λ n X −! Λ ⊕ mi ⊕ j p Λ fj Λ i=1 j=1 whose kernel and cokernel are finite. Proofs can be found in [43], [29, Theorem 5.3.8] or quite different in [50, Theorem 13.12] and [20, Theorem 5.3.1]. The main reason is that Λ is a 2-dimensional local, unique factorisation domain. As the ideals fjΛ and the integers r,. , nt are uniquely determined by X, we can define the following invariants attached to X. The rank of X is rankΛ(X) = r. The Ps Pt µ-invariant is µ(X) = i=1 mi and the λ-invariant is λ(X) = j=1 nj · deg(fj). Finally, if r = 0, t µ(X) Y nj char(X) = p · fj Λ j=1 is called the characteristic ideal of X. If r = 0, s 6 1 and all fj are pairwise coprime, then X ! Λ= char(X) has finite kernel and cokernel. Let us summarise the useful properties of Λ-modules in a lemma. Write XnΓ for the largest quotient of X on which nΓ = Gal(1F=nF ) acts trivially. Lemma 5. Let X be a Λ-module. a). X is finitely generated if and only if X is compact and XΓ is a finitely generated Zp-module. b). Suppose X is a finitely generated Λ-module. Then X is Λ-torsion, i.e., the Λ-rank of X is 0, if and only if XnΓ has bounded Zp-rank. en c). If XnΓ is finite for all n, then there are constants ν and n0 such that jXnΓj = p n with en = µ(X) · p + λ(X) · n + ν for all n > n0. Proofs can be found in x5.3. of [29]. Note that if X = Λ=f for an irreducible f, then n pn XnΓ is finite, unless f is a factor of the distinguished polynomial ! = (1 + T ) − 1 corresponding to a topological generator of nΓ. 1.3 Proof of Iwasawa's theorem I will sketch the proof of theorem 1 only in the simplified case when F has a single prime p above p and that this prime is totally ramified in 1F=F . Let nL be the p-Hilbert class field of nF , i.e. the largest unramified extension of nF whose Galois group is abelian and a p-group. By class field theory the Galois group of nL=nF is isomorphic to nC. 3 Iwasawa theory cw 12 1L Set 1L = S nL, which is a Galois extension of 1F with Galois group X = lim nC. The action of nΓ on nC translates − to an action of Γ on X given by the following. Let γ 2 Γ and X K 9 1 9 x 2 X. Choose a lift g of γ to the Galois group of L=F and 99 γ −1 XΓ 9 set x = gxg . So X is a compact Λ-module. 1 99 F 99 Define K to be the largest abelian extension of F inside 1L. << 9 < 99 Then 0L and 1F are contained in K. The maximality of K << 99 << Γ shows that the Galois group of K=1F must be equal to X . << 0L Γ << Since K=1F is unramified and 1F=F is totally ramified at << ÖÖ < ÖC < ÖÖ p, the inertia group I at a prime above p in K gives a section F of the map from Gal(K=F ) ! Γ. Since KI = 0L, we have Gal(K=1F ) = 0C =: C and it has a trivial action of Γ on it. Hence C is isomorphic to n ∼ n XΓ. Replacing in this argument F by F , we can also conclude that XnΓ = C. In particular, it is always finite. Hence X is a finitely generated torsion Λ-module and lemma 5 c) implies the theorem. Iwasawa has given an example in [16] of a Zp-extension with µ(X) > 0, however he conjectured that µ(X) = 0 whenever the tower is the cyclotomic Zp-extension. This was shown to be true by Ferrero{Washington [11] when F=Q is abelian.

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