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Computing Ray Class Groups, Conductors and Discriminants

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Computing Ray Class Groups, Conductors and Discriminants MATHEMATICS OF COMPUTATION Volume 67, Number 222, April 1998, Pages 773{795 S 0025-5718(98)00912-0 COMPUTING RAY CLASS GROUPS, CONDUCTORS AND DISCRIMINANTS H. COHEN, F. DIAZ Y DIAZ, AND M. OLIVIER Abstract. We use the algorithmic computation of exact sequences of Abelian groups to compute the complete structure of (ZK =m)∗ for an ideal m of a number field K, as well as ray class groups of number fields, and conductors and discriminants of the corresponding Abelian extensions. As an application we give several number fields with discriminants less than previously known ones. The paper is divided as follows. In 1, we give a complete algorithm for com- § puting the groups (ZK /m)∗ for a number field K and an arbitrary modulus m.In 2, we describe the tools necessary for the determination of the ray class group of a number§ field, and also for solving the corresponding principal ideal problem. In 3, we explain how to compute signatures, conductors and discriminants of the fields§ associated to subgroups of the ray class group by global class field theory. In prin- ciple we can give relative and absolute discriminants of all Abelian extensions of a given base field. Finally in 4, we give some numerical examples obtained by these methods. In particular, we§ obtain in this way 10 totally complex number fields of degree less than 80 whose discriminants are less than previously known ones. Using Kummer theory (see e.g. [Da-Po]), we can also obtain a defining equation for these number fields, and we have done this for 9 of the 10 new fields that we have found. We refer to [Ca-Fr] for notation, definitions and results on global class field theory. 1. Computing the structure of (ZK /m)∗ Let K be a number field, and m a modulus in K, in other words m = m0m is a (formal) product of an integral ideal of K and a subset of the set of real places∞ of K.WedenotebyZK the ring of integers of K. In this section, we explain how to compute the group (ZK /m)∗ =(ZK/m0)∗ m × F2∞. The theoretical answer to this question is in principle solved in [Nak]. How- ever this is not suited to algorithmic purposes, and in addition is much more com- plicated than the solution we present below. The natural map from the set of elements of ZK coprime to m to (ZK /m)∗ is surjective. Thus, we could represent an element of this set as the class of an element of ZK (the so-called one-element representation). However we prefer the following representation which is computationally simpler. Received by the editor February 19, 1996 and, in revised form, October 30, 1996. 1991 Mathematics Subject Classification. Primary 11R37, 11Y40. Key words and phrases. Ray class groups, conductors, discriminants . c 1998 American Mathematical Society 773 License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use 774 H. COHEN, F. DIAZ Y DIAZ, AND M. OLIVIER If m = m0m we represent an element in (ZK /m)∗ as a pair (α, v)whereα ∞ m ∈ (ZK/m0)∗ and v F ∞ considered as a column vector. Note that even when m ∈ 2 is an ideal (i.e. m is empty), we still consider pairs (α, v)wherevis the unique ∞ vector in 0-dimensional space over F2.If(α, v) (ZK /m)∗, we will say that α is the finite part,andvthe infinite part or the Archimedean∈ part. The group law in (ZK /m)∗ corresponds to multiplying the finite parts and adding the infinite parts. In all the algorithms that we will present, the above representation is sufficient and simpler than the one-element representation. In some cases, however, it may be desirable to find such a representation. To obtain it, one can do the following: Let m = m0m be a modulus and (α, v) a pair representing an element of (ZK /m)∗, ∞ m with v =(vj)j m and call s the sign homomorphism from ZK to F2 ∞ . Compute ∈ ∞ a Z-basis γ1, ... ,γn of the ideal m0. Considering small linear combinations of the γi, find k elements β1, ... ,βk such that the matrix A over F2 whose columns are 1 the s(βj ) is invertible. Set w = A− v, and let w =(wj). A suitable element is β = α βj. j m w∈Y=0∞ j 6 Since the final β may be large, we may want to reduce it. This cannot be done too rashly however, since we must now preserve the signature of β. We will discuss this at the end of 2. § To compute the structure of (ZK /m)∗, we start by the following lemma, whose non-algorithmic version is trivial. Lemma 1.1. Let a and c be two coprime integral ideals of K. (1) We can algorithmically find elements a a and c c such that a + c =1. (2) Set b = ac. We have a split exact sequence∈ ∈ φ 1 ( Z K / a ) ∗ ( Z K / b ) ∗ ( Z K / c ) ∗ 1 , −→ −→ −→ −→ where ψ(α)=cα + a, φ(β)=βand a section σ of φ is given by σ(γ)= aγ + c.(Here denotes the classes in the respective groups, but using the same notation for each will not lead to any confusion as long as we know in which group we are working.) Proof. (1) By assumption, we have a + c = ZK . Thus there exist a a and c c such that a + c = 1. The algorithm to find them is explained in∈ [Coh2]. For∈ completeness we sketch it here. Let A (resp. C) be the Hermite normal form of a and c with respect to some fixed integral basis of ZK . We assume (it is easy to reduce to this case) that the first element of the integral basis is 1. We apply the Hermite normal form algorithm to the matrix (A C), where here and later, (A C) denotes the (horizontal) concatenation of the matrices| A and C.IfUis a unimodular| matrix obtained during the HNF, we have (A C)U =(0I)whereI | | is the d d identity matrix (where d =[K:Q]) since a and c are coprime. Let × X1 (resp. X2) be the top (resp. bottom) half of the (d +1)stcolumnofU.Then AX1 and CX2 represent, with respect to the fixed integral basis, elements a and c satisfying the required conditions. The proof of (2) is straightforward and tedious, and left to the reader. Remark. More generally, if a and c are two coprime moduli (i.e. a0 + c0 = ZK and a c = ) there exist elements a and c such that a + c =1,a 1(mod∗c), ∞ ∩ ∞ ∅ ≡ c 1(mod∗a). There is also an exact sequence generalizing (2). ≡ License or copyright restrictions may apply to redistribution; see https://www.ams.org/journal-terms-of-use COMPUTING RAY CLASS GROUPS, CONDUCTORS AND DISCRIMINANTS 775 A finitely generated Abelian group will be represented by generators and re- G lations, i.e. as a pair (G, DG)whereGis a (finite) set of generators of G given as a row vector, and DG is a matrix representing a set of relations for G in Smith Normal Form. Since the Abelian groups that we will use are multiplicative, it is to be understood that a product such as GX (where X is a column vector of integers) is to be interpreted in a multiplicative sense, i.e. if G =(gi)andX=(xi), then xi GX = gi . Thus, let (A, DA)(resp.(C, DC )) be the Smith Normal Form of (ZK /a)∗ (resp. Q (ZK /c)∗). By Lemma 1.1, it is clear that to obtain the structure of (ZK /b)∗ we can simply construct a diagonal matrix using DA and DC as diagonal blocks, and then transform this matrix into its Smith Normal Form. More generally, we will need to perform algorithmically standard operations on Abelian groups such as computing kernels, images, quotients and extensions. Since it is not easy to find explicit descriptions of these algorithms in the literature, and since some details are not completely trivial, we give some explanations for the most important case of group extensions (see [Co-Di-Ol] for complete details). Let =(A, DA)and =(C, DC ) be given in SNF. We assume that we can solve the discreteA logarithm problemC in and , in other words, that we can express an arbitrary element of the groups asA a powerC product of its generators. Now assume that we have an exact sequence φ 1 1 , −→ A −→ B −→ C −→ and we want to compute the SNF (B,DB) of the group . This is done using the following proposition. B Proposition 1.2. With the above notation, let B0 be such that φ(B0)=C,set S=(B0 ψ(A)). | (1) There exists a matrix P with integer coefficients such that B0DC = ψ(A)P . (2) Set D 0 M = C . PDA − Then (S,M) is a system of generators and relations for the group , and hence B we can obtain the SNF (B,DB) of the group by applying the Smith normal formalgorithmto(S, M). B Proof. Since φ is surjective, we can find B0 such that φ(B0)=C.Letβ . ∈B We can write φ(β )=CX = φ(B0)X = φ(B0X), hence β B0X Ker(φ)so − ∈ β B0X=ψ(AY ) for some integer vector Y .Thusβ=B0X+ψ(A)Y=(B0 ψ(A))R − X | where R = Y is the vertical concatenation of the column vectors X and Y .It follows that S =(B0 ψ(A)) forms a generating set for .
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