DOCSLIB.ORG

Class Number Formula for Dihedral Extensions 2

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Class Number Formula for Dihedral Extensions 2 CLASS NUMBER FORMULA FOR DIHEDRAL EXTENSIONS LUCA CAPUTO AND FILIPPO A. E. NUCCIO MORTARINO MAJNO DI CAPRIGLIO Abstract. We give an algebraic proof of a class number formula for dihedral extensions of number fields of degree 2q, where q is any odd integer. Our formula expresses the ratio of class numbers as a ratio of orders of cohomology groups of units and allows one to recover similar formulas which have appeared in the literature. As a corollary of our main result we obtain explicit bounds on the (finitely many) possible values which can occur as ratio of class numbers in dihedral extensions. Such bounds are obtained by arithmetic means, without resorting to deep integral representation theory. 1. Introduction Let k be a number field and let M/k be a finite Galois extension. For a subgroup H Gal(M/k), let χH denote the rational character of the permutation representation defined by the Gal(M/k)-set⊆ Gal(M/k)/H. If M/k is non-cyclic, then there exists a relation n χ =0 H · H XH where nH Z are not all 0 and H runs through the subgroups of Gal(M/k). Using the formalism of Artin L-functions∈ and the formula for the residue at 1 of the Dedekind zeta-function, Brauer showed in [Bra51] that the above relation translates into the following formula nH H nH wM (1.1) (hM H ) = RM H YH YH where, for a number field E, hE, RE, wE denote the class number, the regulator and the order of the group of roots of unity of E, respectively. Although the regulator of a number field is in general an irrational number, the right-hand side of the above equality is clearly rational, being equal to the left-hand side. As a matter of fact, Brauer himself proved that when k = Q the regulator can be translated into an algebraic invariant of the Galois structure of the unit group. It is therefore natural to look for an algebraic proof of Brauer’s formula, independent of any analytical result. Brauer also showed that the left-hand side of (1.1) can take only finitely many values as M ranges over all arXiv:1803.04064v2 [math.NT] 8 May 2019 Galois extension of Q with Galois group isomorphic to a fixed finite group. It thus becomes interesting to look for an explicit description of a finite set of integers (as small as possible) containing all the values which are attainable by the class number ratio, as well as for generalizations beyond the case k = Q. Historically, these questions were investigated for the first time in the case where M = Q(√d, √ d) (with d> 1 a square-free integer). In this case Dirichlet showed, by analytical means, that − h 1 M = Q hQ(√d)hQ(√ d) 2 − where Q is 1 or 2 depending on whether a certain unit of M has norm 1 or not in Q(√ 1) (see [Dir42]). Hilbert later gave an algebraic proof of the above equality (see [Hil94] and [Lem94] for a more− general result). Date: May 9, 2019. 2010 Mathematics Subject Classification. 11R29, 11R20, 11R34, 11R37. 1 CLASS NUMBER FORMULA FOR DIHEDRAL EXTENSIONS 2 Another interesting particular case is that of dihedral extensions, which is the one we focus on in this paper. This setting has already been considered by other authors: notably, Walter computed explicitly in [Wal77] the regulator term, building upon Brauer’s previous work and obtained a refined class number formula in the dihedral case over an arbitrary base field k, still relying on analytical results about Artin L-functions (Walter’s result actually holds for more general Frobenius groups). Bartel provided a different representation-theoretic interpretation of Walter’s result in [Bar12], a result which was later generalized to a much larger family of groups in [BdS13]. Our main result is the following theorem whose proof is purely algebraic and does not rely on any argument involving L-functions. Theorem 3.14. Let L/k be a Galois extension of number fields whose Galois group D = Gal(L/k) is dihedral of order 2q with q odd. Let Σ D be a subgroup of order 2 and G D be the subgroup of order q. Set K = LΣ and F = LG. Then ⊆ ⊆ 2 0 1 0 1 h h h D, × Z h (D, ×) h− (Σ, ×) L k = OL ⊗ 2 = OL OL . 2 1 hF h 1 1 0 K h− D, L× Z 2 h− (D, L×) h (Σ, L×) b O ⊗ b O b O i Here L× is the group of units of bL and, for a subgroup Hb D and anb integer i, h (H,B) denotes the order of the i-thO Tate cohomology group of H with values in the H⊆-module B. Similar algebraic proofs have appeared in the literature although, as far as we know,b only for special cases. Halter-Koch in [HK77] and Moser in [Mos79] analysed the case of a dihedral extension of Q of degree 2p, where p is an odd prime. They express the ratio of class numbers in terms of a unit index, proving that in this setting one has hL r 2 = L× : K× K× ′ F× p− hF hK O O O O where K′ is a Galois conjugate of K in L, andr = 1 if F is imaginary and 2 otherwise. Their result was generalized by Jaulent in [Jau81], who gave an algebraic proof of a similar formula for dihedral extensions of degree 2ps when the ground field k is Q or an imaginary quadratic field and s 1 is any integer. Jaulent actually obtained his result as a particular case of a formula for Galois extensions over≥ an arbitrary ground field with Galois group of the form Z/ps ⋊ T where T is an abelian group of exponent dividing p 1. In such general formula the ratio of class numbers is expressed in terms of orders of cohomology groups− of units, as in our Theorem 3.14. This approach was not developed further and in fact Jaulent’s paper is not cited in subsequent works in the literature. The study of class number formulas for dihedral extensions from an algebraic viewpoint was taken up more than 20 years later by Lemmermeyer. In [Lem05], he generalized Halter-Koch’s formula to arbitrary ground fields, always under the assumption that the degree is 2p and under some ramification condition. The quoted works also provide an explicit description of the finitely many possible values which may occur as class number ratio, clearly only under their working assumptions. These values are obtained using the explicit classification of the indecomposable integral representations of D, in order to find an exhaustive list of the possible indexes of units which can appear as regulator ratios. This classification is not available for dihedral groups of arbitrary order 2q, as discussed in the introduction of [HR63], for instance. Combining our formula with the knowledge of the Herbrand quotient of the units for the cyclic subextension L/F , we find a finite set of integers containing all the attainable values of the ratio of class numbers, without resorting to any integral representation theoretic results. This indicates that the traditional way of expressing the class number ratio as a unit index, although suggestive, is less effective than our cohomological approach for the purpose of determining the possible values of the ratio of class numbers. Our main results in this direction are the following two corollaries. For a number field M, denote by µM (q∞) the roots of unity in M of q-power order, and by µM (q) the subgroup of µM (q∞) of elements killed by q. Set 0 if µM (q) is trivial, βM (q)= (1 otherwise CLASS NUMBER FORMULA FOR DIHEDRAL EXTENSIONS 3 Give a a prime number ℓ, vℓ denotes the ℓ-adic valuation. Corollary 3.15. Let a = rkZ( F×)+βF (q)+1 and b = rkZ( k×)+βk(q). Then, for every prime ℓ, the following bounds hold O O h h2 av (q) v L k bv (q). − ℓ ≤ ℓ h h2 ≤ ℓ F K In particular, if F is totally real of degree [F : Q]=2d, then h h2 2dv (q) v L k (d 1)v (q). − ℓ ≤ ℓ h h2 ≤ − ℓ F K In case F is assumed to be a CM field and k = F + is its totally real subfield, we can strengthen our bounds as follows. Corollary 3.18. Assume that F is a CM field of degree [F : Q]=2d and that k is its totally real subfield. Let q q s be the order of the quotient ( K× ) k× /( k×) and let t be the order of µF (q∞)/µF (q). Then, for every prime ℓ, O ∩ O O h h2 v (q) v (s) v (t) v L k (d 1)v (q). − ℓ − ℓ − ℓ ≤ ℓ h h2 ≤ − ℓ F K When q = p is prime and k = Q, we can further improve Corollary 3.15 and prove in Corollary 3.20 that the ratio of class numbers satisfies h 1 1 L 1, , . h h2 ∈ p p2 F K This answers the MathOverflow question [Bar] of Bartel, who asks for a proof of this fact that does not rely on the analytical class number formula, nor on any integral representation theory. Our Corollary 3.15 actually shows that a similar result holds without the assumption that k = Q or that q = p be prime. We now briefly describe the structure of the paper.
Load more

Recommended publications
  • An Introduction to Iwasawa Theory
    An Introduction to Iwasawa Theory Notes by Jim L. Brown 2 Contents 1 Introduction 5 2 Background Material 9 2.1 AlgebraicNumberTheory . 9 2.2 CyclotomicFields........................... 11 2.3 InfiniteGaloisTheory . 16 2.4 ClassFieldTheory .......................... 18 2.4.1 GlobalClassFieldTheory(ideals) . 18 2.4.2 LocalClassFieldTheory . 21 2.4.3 GlobalClassFieldTheory(ideles) . 22 3 SomeResultsontheSizesofClassGroups 25 3.1 Characters............................... 25 3.2 L-functionsandClassNumbers . 29 3.3 p-adicL-functions .......................... 31 3.4 p-adic L-functionsandClassNumbers . 34 3.5 Herbrand’sTheorem . .. .. .. .. .. .. .. .. .. .. 36 4 Zp-extensions 41 4.1 Introduction.............................. 41 4.2 PowerSeriesRings .......................... 42 4.3 A Structure Theorem on ΛO-modules ............... 45 4.4 ProofofIwasawa’stheorem . 48 5 The Iwasawa Main Conjecture 61 5.1 Introduction.............................. 61 5.2 TheMainConjectureandClassGroups . 65 5.3 ClassicalModularForms. 68 5.4 ConversetoHerbrand’sTheorem . 76 5.5 Λ-adicModularForms . 81 5.6 ProofoftheMainConjecture(outline) . 85 3 4 CONTENTS Chapter 1 Introduction These notes are the course notes from a topics course in Iwasawa theory taught at the Ohio State University autumn term of 2006. They are an amalgamation of results found elsewhere with the main two sources being [Wash] and [Skinner]. The early chapters are taken virtually directly from [Wash] with my contribution being the choice of ordering as well as adding details to some arguments. Any mistakes in the notes are mine. There are undoubtably type-o’s (and possibly mathematical errors), please send any corrections to [email protected]. As these are course notes, several proofs are omitted and left for the reader to read on his/her own time.
    [Show full text]
  • Automorphic Functions and Fermat's Last Theorem（3）
    Jiang and Wiles Proofs on Fermat Last Theorem (3) Abstract D.Zagier(1984) and K.Inkeri(1990) said[7]:Jiang mathematics is true,but Jiang determinates the irrational numbers to be very difficult for prime exponent p.In 1991 Jiang studies the composite exponents n=15,21,33,…,3p and proves Fermat last theorem for prime exponenet p>3[1].In 1986 Gerhard Frey places Fermat last theorem at the elliptic curve that is Frey curve.Andrew Wiles studies Frey curve. In 1994 Wiles proves Fermat last theorem[9,10]. Conclusion:Jiang proof(1991) is direct and simple,but Wiles proof(1994) is indirect and complex.If China mathematicians had supported and recognized Jiang proof on Fermat last theorem,Wiles would not have proved Fermat last theorem,because in 1991 Jiang had proved Fermat last theorem.Wiles has received many prizes and awards,he should thank China mathematicians. - Automorphic Functions And Fermat’s Last Theorem（3） (Fermat’s Proof of FLT) 1 Chun-Xuan Jiang P. O. Box 3924, Beijing 100854, P. R. China [email protected] Abstract In 1637 Fermat wrote: “It is impossible to separate a cube into two cubes, or a biquadrate into two biquadrates, or in general any power higher than the second into powers of like degree: I have discovered a truly marvelous proof, which this margin is too small to contain.” This means: xyznnnn+ =>(2) has no integer solutions, all different from 0(i.e., it has only the trivial solution, where one of the integers is equal to 0). It has been called Fermat’s last theorem (FLT).
    [Show full text]
  • Introduction to L-Functions: Dedekind Zeta Functions
    Introduction to L-functions: Dedekind zeta functions Paul Voutier CIMPA-ICTP Research School, Nesin Mathematics Village June 2017 Dedekind zeta function Definition Let K be a number field. We define for Re(s) > 1 the Dedekind zeta function ζK (s) of K by the formula X −s ζK (s) = NK=Q(a) ; a where the sum is over all non-zero integral ideals, a, of OK . Euler product exists: Y −s −1 ζK (s) = 1 − NK=Q(p) ; p where the product extends over all prime ideals, p, of OK . Re(s) > 1 Proposition For any s = σ + it 2 C with σ > 1, ζK (s) converges absolutely. Proof: −n Y −s −1 Y 1 jζ (s)j = 1 − N (p) ≤ 1 − = ζ(σ)n; K K=Q pσ p p since there are at most n = [K : Q] many primes p lying above each rational prime p and NK=Q(p) ≥ p. A reminder of some algebraic number theory If [K : Q] = n, we have n embeddings of K into C. r1 embeddings into R and 2r2 embeddings into C, where n = r1 + 2r2. We will label these σ1; : : : ; σr1 ; σr1+1; σr1+1; : : : ; σr1+r2 ; σr1+r2 . If α1; : : : ; αn is a basis of OK , then 2 dK = (det (σi (αj ))) : Units in OK form a finitely-generated group of rank r = r1 + r2 − 1. Let u1;:::; ur be a set of generators. For any embedding σi , set Ni = 1 if it is real, and Ni = 2 if it is complex. Then RK = det (Ni log jσi (uj )j)1≤i;j≤r : wK is the number of roots of unity contained in K.
    [Show full text]
  • Notes on the Riemann Hypothesis Ricardo Pérez-Marco
    Notes on the Riemann Hypothesis Ricardo Pérez-Marco To cite this version: Ricardo Pérez-Marco. Notes on the Riemann Hypothesis. 2018. hal-01713875 HAL Id: hal-01713875 https://hal.archives-ouvertes.fr/hal-01713875 Preprint submitted on 21 Feb 2018 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. NOTES ON THE RIEMANN HYPOTHESIS RICARDO PEREZ-MARCO´ Abstract. Our aim is to give an introduction to the Riemann Hypothesis and a panoramic view of the world of zeta and L-functions. We first review Riemann's foundational article and discuss the mathematical background of the time and his possible motivations for making his famous conjecture. We discuss some of the most relevant developments after Riemann that have contributed to a better understanding of the conjecture. Contents 1. Euler transalgebraic world. 2 2. Riemann's article. 8 2.1. Meromorphic extension. 8 2.2. Value at negative integers. 10 2.3. First proof of the functional equation. 11 2.4. Second proof of the functional equation. 12 2.5. The Riemann Hypothesis. 13 2.6. The Law of Prime Numbers. 17 3. On Riemann zeta-function after Riemann.
    [Show full text]
  • And Siegel's Theorem We Used Positivi
    Math 259: Introduction to Analytic Number Theory A nearly zero-free region for L(s; χ), and Siegel's theorem We used positivity of the logarithmic derivative of ζq to get a crude zero-free region for L(s; χ). Better zero-free regions can be obtained with some more effort by working with the L(s; χ) individually. The situation is most satisfactory for 2 complex χ, that is, for characters with χ = χ0. (Recall that real χ were also the characters that gave us the most difficulty6 in the proof of L(1; χ) = 0; it is again in the neighborhood of s = 1 that it is hard to find a good zero-free6 region for the L-function of a real character.) To obtain the zero-free region for ζ(s), we started with the expansion of the logarithmic derivative ζ 1 0 (s) = Λ(n)n s (σ > 1) ζ X − − n=1 and applied the inequality 1 0 (z + 2 +z ¯)2 = Re(z2 + 4z + 3) = 3 + 4 cos θ + cos 2θ (z = eiθ) ≤ 2 it iθ s to the phases z = n− = e of the terms n− . To apply the same inequality to L 1 0 (s; χ) = χ(n)Λ(n)n s; L X − − n=1 it it we must use z = χ(n)n− instead of n− , obtaining it 2 2it 0 Re 3 + 4χ(n)n− + χ (n)n− : ≤ σ Multiplying by Λ(n)n− and summing over n yields L0 L0 L0 0 3 (σ; χ ) + 4 Re (σ + it; χ) + Re (σ + 2it; χ2) : (1) ≤ − L 0 − L − L 2 Now we see why the case χ = χ0 will give us trouble near s = 1: for such χ the last term in (1) is within O(1) of Re( (ζ0/ζ)(σ + 2it)), so the pole of (ζ0/ζ)(s) at s = 1 will undo us for small t .
    [Show full text]
  • Arxiv:1802.06193V3 [Math.NT] 2 May 2018 Etr ...I Hsppr Efi H Udmna Discrimi Fundamental the fix Fr We Quadratic Paper, Binary I This a Inverse by in the Squares 1.4.]
    THE LEAST PRIME IDEAL IN A GIVEN IDEAL CLASS NASER T. SARDARI Abstract. Let K be a number field with the discriminant DK and the class number hK , which has bounded degree over Q. By assuming GRH, we prove that every ideal class of K contains a prime ideal with norm less 2 2 than hK log(DK ) and also all but o(hK ) of them have a prime ideal with 2+ǫ norm less than hK log(DK ) . For imaginary quadratic fields K = Q(√D), by assuming Conjecture 1.2 (a weak version of the pair correlation conjecure), we improve our bounds by removing a factor of log(D) from our bounds and show that these bounds are optimal. Contents 1. Introduction 1 1.1. Motivation 1 1.2. Repulsion of the prime ideals near the cusp 5 1.3. Method and Statement of results 6 Acknowledgements 7 2. Hecke L-functions 7 2.1. Functional equation 7 2.2. Weil’s explicit formula 8 3. Proof of Theorem 1.8 8 4. Proof of Theorem 1.1 11 References 12 1. Introduction 1.1. Motivation. One application of our main result addresses the optimal upper arXiv:1802.06193v3 [math.NT] 2 May 2018 bound on the least prime number represented by a binary quadratic form up to a power of the logarithm of its discriminant. Giving sharp upper bound of this form on the least prime or the least integer representable by a sum of two squares is crucial in the analysis of the complexity of some algorithms in quantum compiling.
    [Show full text]
  • Asymptotic Problems in Number Theory Summary of Lectures- Spring 2015
    Math 719: Asymptotic Problems in Number Theory Summary of Lectures- Spring 2015. In this document I will give a summary of what we have covered so far in the course, provide references, and given some idea of where we are headed next. 1 Lecture 1 In the first lecture I gave an overview of what we will cover in the first half of the course. The broad goal is to understand the behavior of families of arithmetic objects. We want to understand what an `average' object looks like, and also what an `extremal' object looks like. We will look at this question in a few different settings. The three main topics will (likely) be: 1. Class groups of imaginary quadratic fields; 2. Curves over finite fields; 3. Linear codes. Our first major goal is to understand positivep definite quadratic forms of given discriminant and then connection to class groups of the field Q( −d), or really more accurately, the quadratic ring of discriminant −d. We will spend approximately the first three weeks setting up this connection, first focusing on quadratic forms and then transferring over to ideal class groups. p As we vary over all quadratic imaginary fields K = Q( −d) where d > 0 is squarefree, we get some set of finite abelian groups C(OK ). What can we say about these groups? How large are they on average? How often is this group trivial? What can we say about the p-parts of these groups for fixed p? Understanding these questions will require using some of the key ideas of class field theory and also understanding some analytic techniques.
    [Show full text]
  • The Class Number Formula for Quadratic Fields and Related Results
    The Class Number Formula for Quadratic Fields and Related Results Roy Zhao January 31, 2016 1 The Class Number Formula for Quadratic Fields and Related Results Roy Zhao CONTENTS Page 2/ 21 Contents 1 Acknowledgements 3 2 Notation 4 3 Introduction 5 4 Concepts Needed 5 4.1 TheKroneckerSymbol.................................... 6 4.2 Lattices ............................................ 6 5 Review about Quadratic Extensions 7 5.1 RingofIntegers........................................ 7 5.2 Norm in Quadratic Fields . 7 5.3 TheGroupofUnits ..................................... 7 6 Unique Factorization of Ideals 8 6.1 ContainmentImpliesDivision................................ 8 6.2 Unique Factorization . 8 6.3 IdentifyingPrimeIdeals ................................... 9 7 Ideal Class Group 9 7.1 IdealNorm .......................................... 9 7.2 Fractional Ideals . 9 7.3 Ideal Class Group . 10 7.4 MinkowskiBound....................................... 10 7.5 Finiteness of the Ideal Class Group . 10 7.6 Examples of Calculating the Ideal Class Group . 10 8 The Class Number Formula for Quadratic Extensions 11 8.1 IdealDensity ......................................... 11 8.1.1 Ideal Density in Imaginary Quadratic Fields . 11 8.1.2 Ideal Density in Real Quadratic Fields . 12 8.2 TheZetaFunctionandL-Series............................... 14 8.3 The Class Number Formula . 16 8.4 Value of the Kronecker Symbol . 17 8.5 UniquenessoftheField ................................... 19 9 Appendix 20 2 The Class Number Formula for Quadratic Fields and Related Results Roy Zhao Page 3/ 21 1 Acknowledgements I would like to thank my advisor Professor Richard Pink for his helpful discussions with me on this subject matter throughout the semester, and his extremely helpful comments on drafts of this paper. I would also like to thank Professor Chris Skinner for helping me choose this subject matter and for reading through this paper.
    [Show full text]
  • Dirichlet's Class Number Formula
    Dirichlet's Class Number Formula Luke Giberson 4/26/13 These lecture notes are a condensed version of Tom Weston's exposition on this topic. The goal is to develop an analytic formula for the class number of an imaginary quadratic field. Algebraic Motivation Definition.p Fix a squarefree positive integer n. The ring of algebraic integers, O−n ≤ Q( −n) is defined as ( p a + b −n a; b 2 Z if n ≡ 1; 2 mod 4 O−n = p a + b −n 2a; 2b 2 Z; 2a ≡ 2b mod 2 if n ≡ 3 mod 4 or equivalently O−n = fa + b! : a; b 2 Zg; where 8p −n if n ≡ 1; 2 mod 4 < p ! = 1 + −n : if n ≡ 3 mod 4: 2 This will be our primary object of study. Though the conditions in this def- inition appear arbitrary,p one can check that elements in O−n are precisely those elements in Q( −n) whose characteristic polynomial has integer coefficients. Definition. We define the norm, a function from the elements of any quadratic field to the integers as N(α) = αα¯. Working in an imaginary quadratic field, the norm is never negative. The norm is particularly useful in transferring multiplicative questions from O−n to the integers. Here are a few properties that are immediate from the definitions. • If α divides β in O−n, then N(α) divides N(β) in Z. • An element α 2 O−n is a unit if and only if N(α) = 1. • If N(α) is prime, then α is irreducible in O−n .
    [Show full text]
  • Department of Mathematics University of Notre Dame
    Department of Mathematics University of Notre Dame GRADUATE STUDENT SEMINAR Guest Speaker: Eric Wawerczyk University of Notre Dame Date: Monday, October 3, 2016 Time: 4:00 PM Location: 129 Hayes-Healy Hall Lecture Title: Dreams of a Number Theorist: The left side of the right-half- plane Abstract Euler studied the Harmonic series, the sums of reciprocals of squares, cubes, and so on. Chebyshev was able to package these together in the zeta function, defined for a real number greater than 1. Riemann had the idea of defining the function for complex numbers and found that this infinite series converges as long as the real part of the imaginary number is greater than 1. He then uses the inverse mellin transform of the Theta Function (see: modular forms) to derive the "functional equation" which gives a UNIQUE meromorphic extension to the whole complex plane outside of the point s=1 (harmonic series diverges=pole). This unique extension is called the Riemann Zeta Function. For over 150 years Number Theorists have been staring at the left side of the right half plane. We will explore this mysterious left hand side throughout the talk and its relationship to Number Theory. This will serve as our prototypical example of Zeta Functions. Finally, we will discuss generalizations of the zeta functions (Dedekind Zeta Functions, Artin Zeta Functions, Hasse-Weil Zeta Functions) and our daunting task of extending all of them to their own left sides and what mysteries lie within (The Analytic Class Number Formula, The (proven) Weil Conjectures, the Birch and Swinnerton-Dyer conjecture, The Langlands Program, and the (proven) Taniyama-Shimura Conjecture, the Grand Riemann Hypothesis ).
    [Show full text]
  • Introduction to Iwasawa Theory
    Topics in Number Theory Introduction to Iwasawa Theory 8 David Burns Giving a one-lecture-introduction to Iwasawa theory is an unpossibly difficult task as this requires to give a survey of more than 150 years of development in mathematics. Moreover, Iwasawa theory is a comparatively technical subject. We abuse this as an excuse for missing out the one or other detail. 8.1 The analytic class number formula We start our journey with 19th-century-mathematics. The reader might also want to consult his notes on last week’s lecture for a more extensive treatment. Let R be an integral domain and F its field of fractions. A fractional ideal I of R is a finitely generated R-submodule of F such that there exists an x 2 F× satisfying x · I ⊆ R. For such a fractional ideal I we define its dual ideal to be I∗ = fx 2 F j x · I ⊆ Rg. If I and J both are fractional ideals, its product ideal I · J is given by the ideal generated by all products i · j for i 2 I and j 2 J. That is, I · J = f ∑ ia · ja j ia 2 I, ja 2 J, A finite g. a2A For example we have I · I∗ ⊆ R. If I · I∗ = R we say that I is invertible. We now specialise to R being a Dedekind domain. In this case, every non-zero fractional ideal is invertible. Hence the set of all nonzero fractional ideals of R forms an abelian group under multiplication which we denote by Frac(R).
    [Show full text]
  • The Analytic Class Number Formula
    The Analytic Class Number Formula Gary Sivek [email protected] May 19, 2005 1 Introduction In this paper we will use tools from analysis to provide an explicit formula for the class number of a number field. We will then examine an approach to evaluating the formula in the case of a quadratic field. This paper assumes basic knowledge of number theory and only minimal complex analysis (what it means for a function to be analytic, how to find residues, and what an L-series is, including the zeta function). For an introduction to complex analysis, see for example [Ahl79], and for an introduction to algebraic number theory, read [Ste05]. In particular, we assume an understanding of the finiteness of the class group of a number field and concepts related to its proof. 2 The Formula Here we prove the class number formula, which puts the class number of a number field in terms of its zeta function. The formula was originally proven in terms of the number of binary quadratic forms with a given determinant by Dirichlet, and was later proven for general number fields by Dedekind [Hil97]. 2.1 Definitions From this point on, K will be a number field of degree n = [K : Q] with ring of integers K. The class group of K will be denoted . The units contained in will form a groupO of CK OK size ωK, and K will have discriminant DK . An ideal i K will have norm N(i), which will be understood to be the K/Q norm. ⊆ O Pick any number field K with S real embeddings σ ,...,σ and T = 1 (n S) pairs of 1 S 2 − complex embeddings τ1, τ1,...,τT , τT .
    [Show full text]