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The Analytic Class Number Formula for Orders in Products of Number Fields

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The Analytic Class Number Formula for Orders in Products of Number Fields THE ANALYTIC CLASS NUMBER FORMULA FOR 1-DIMENSIONAL AFFINE SCHEMES BRUCE W. JORDAN AND BJORN POONEN Abstract. We derive an analytic class number formula valid for an order in a product of S-integers in global fields, or equivalently for reduced finite-type affine schemes of pure dimension 1 over Z. 1. Introduction Let K be a finite extension of Q. Let be its ring of integers. Let ζ (s) be the Dedekind OK K zeta function of K, which is the zeta function of Spec K . Dedekind [Dir94, Supplement XI, 184, IV], generalizing work of Dirichlet, proved theOanalytic class number formula, which § expresses the residue of ζK(s) at s = 1 in terms of arithmetic invariants (see also Hilbert’s Zahlbericht [Hil97, Theorem 56]). More precisely, he proved that 2r1 (2π)r2 hR (1) lim(s 1)ζK(s)= , s→1 − w Disc K 1/2 | | where r1 is the number of real places, r2 is the number of complex places, h is the class number, R is the unit regulator, w is the number of roots of unity, and Disc K is the discriminant. F. K. Schmidt [Sch31, Satz 21] proved an analogue for a global function field. This could be generalized in several ways: Replace by a non-maximal order. • OK Replace K by a ring of S-integers for some finite set S of places of K. • Allow Z-algebrasO of Krull dimension 1 that are not necessarily integral domains. • We will generalize simultaneously in all of these directions, by proving a version of (1) for an order in a product of S-integers in global fields, or equivalently for a reduced affine finite-typeOZ-scheme of pure dimension 1 (for the equivalence, see Proposition 4.2). Our main result, expressing the leading term of the arithmetic zeta function of [Ser65, p. 83] in arXiv:1604.04564v3 [math.NT] 30 Dec 2019 terms of quantities to be defined in Section 5, is this: Theorem 1.1 (Generalized analytic class number formula). Let X be a reduced affine finite- type Z-scheme of pure dimension 1, say X = Spec . Let m be the number of irreducible components of X. Then O r1 r2 −1 m 2 (2π) h( ) R( ) v∈Snonarch ((1 qv ) / log qv) (2) lim(s 1) ζX (s)= O O − . s→1 − w( ) Disc 1/2 OQ| O| Date: December 30, 2019. 2010 Mathematics Subject Classification. Primary 11R54; Secondary 11R29. The second author was supported in part by National Science Foundation grant DMS-1069236 and DMS- 1601946 and grants from the Simons Foundation (#340694, #402472, and #550033). 1 Remark 1.2. Theorem 1.1 seems to be new even for orders in rings of integers of number fields and for rings of S-integers in number fields. Remark 1.3. Replacing a finite-type Z-scheme X by its associated reduced subscheme Xred does not change ζX (s). On the other hand, if the scheme X = Spec in Theorem 1.1 is not assumed to be reduced, then × need not be finitely generated, soO defining the regulator O 2 × R( ) is problematic: for example, if := F2[t, ǫ]/(ǫ ), then is isomorphic to the additive groupO of F [t] via the homomorphismO 1 + fǫ f. O 2 7→ Remark 1.4. Various authors defined other zeta functions attached to a singular curve over a finite field and computed their leading terms at s = 0 or s = 1 [Gal73, Gre89, ZG97, ZG97b, St¨o98], but these zeta functions are different from the usual zeta function in [Ser65] in general. 1.1. Outline of the article. Section 2 proves Theorem 1.1 for a ring of S-integers. Because we want a formula involving the S-class group and S-regulator instead of the usual class group and regulator, it is not convenient to deduce this from the classical formula for the ring of integers. Instead we redo the calculations of Tate’s thesis for S-integers. Sections 3 and 4 characterize the rings such that Spec is a reduced affine finite-type O O Z-scheme of pure dimension 1. In particular, the normalization of such a ring is a product of rings of S-integers. O O Section 5 defines all the quantities that appear in (2). e Section 6 proves Theorem 1.1. The formula for follows from the case proved in Section 2, O so our strategy is to determine how each term in (2) changes when is replaced by . In particular, we use the Leray spectral sequence toe determine how theO unit group and PicardO group change. e Finally Sections 7 and 8 illustrate (2) in examples exhibiting the phenomena that can arise in our context: both number fields and function fields, S-integers instead of just integers, multiple irreducible components, and non-maximal orders (and hence singular points of the scheme). 2. Tate’s thesis for S-integers Let K be a global field. Let µ be the torsion subgroup of K×, and let w = #µ. For each place v of K, let K be the completion of K at v. If K R, equip it with Lebesgue v v ≃ measure. If Kv C, equip it with 2 times Lebesgue measure. For each nonarchimedean v, let be the valuation≃ ring in K , let q be the size of its residue field, and equip K with Ov v v v the Haar measure dx for which vol( v) = 1; here we follow [Wei67, p. 95] instead of taking the self-dual measure as in [Tat67, p.O 310]. We write vol(T ) for the measure of a set T with respect to a measure that is implied by context. If a Kv for some v, let a v R≥0 be the factor by which multiplication-by-a scales the ∈ | | ∈ × Haar measure on Kv. The measure we use on Kv is not the restriction of the measure dx on × Kv. If v is archimedean, equip Kv with the Haar measure dx/ x v. If v is nonarchimedean, × × | | equip Kv with the Haar measure for which vol( v ) = 1. × × O × × Let Kv,1 := x Kv : x v =1 . If v is nonarchimedean, Kv,1 = v , which has volume 1 { ∈ | |× } × O for the Haar measure on Kv . If v is archimedean, then equip Kv,1 with the Haar measure 2 × compatible with the Haar measure on Kv and Lebesgue measure on R in the exact sequence log || 1 K× K× v R 0 −→ v,1 −→ v −→ −→ (for the notion of compatibility, see, e.g., [DE14, Theorem 1.53]; the notion can be extended to exact sequences of arbitrary finite length by breaking them into short exact sequences). ′ Define the ad`ele ring A as the restricted product v(Kv, v) with the product measure. × ′ × × O Equip the id`ele group A = v(Kv , v ) with the product of the multiplicative Haar mea- sures. The field K embeds diagonallyO in A, and K×Qembeds diagonally in A×. Equip the discrete groups K and K× andQ their subgroups with the counting measure, in order to equip A/K and A×/K× with measures. Let S be a finite nonempty set of places of K containing all the archimedean places. Let Snonarch be the set of nonarchimedean places in S. Define the ring of S-integers by := x K : v(x) 0 for all v / S . O { ∈ ≥ ∈ } Lemma 2.1. We have a measure-compatible isomorphism K A v∈S v × v∈ /S Ov ∼ −→ K Q Q and a measure-compatible short exact sequenceO K K 0 v∈S v × v∈ /S Ov v∈S v 0. −→ Ov −→ −→ −→ v∈ /S Q Q Q Y O O Proof. Since S is nonempty, strong approximation [Cas67, 15] implies that any x A can § ∈ be written as y + ǫ with y K and ǫ = (ǫv) A such that ǫv v for all v / S. In other words, the homomorphism ∈ ∈ ∈ O ∈ A K v × Ov −→ K v∈S Y vY∈ /S is surjective. Its kernel is K v∈S Kv v∈ /S v = , so we obtain the claimed isomor- phism. By definition of the measures,∩ the× upperO three homomorphisO ms in the diagram Q Q K / A v∈S v × v∈ /S Ov Q Q K A v∈S v × v∈ /S Ov ∼ / K Q O Q respect the measures, so the induced isomorphism at the bottom does too. The exact sequence is obtained from the measure-compatible split exact sequence 0 K K 0 −→ Ov −→ v × Ov −→ v −→ vY∈ /S vY∈S vY∈ /S vY∈S by dividing each of the last two terms by the image of with the counting measure. O Define the S-Arakelov divisor group (cf. the usual Arakelov divisor group in [Neu99, III.1.8]) by Div := R (Z log q ), O × v v∈S v∈ /S M 3 M d where each R has Lebesgue measure and each Z log qv has the counting measure. We have a 0 homomorphism deg: Div R that sums the components, and its kernel is denoted Div , O→ 0 O which acquires a measure compatible with the sequence 0 Div Div R 0. d× → O → O → d→ The homomorphism A Div sending (xv) to (log xv v) restricts to a homomorphism 0 → O | | 0 K× Div whose cokernel is called the S-Arakelov class groupdPic . LetdA× be the kernel → O O 1 × d deg S of the composition A Div R. Equip R := v∈S R and R with Lebesgue measure. d → O → sum c Equip the sum-zero hyperplane RS := ker RS R with the measure compatible with 0 −→ L d S those (equivalently, identify R0 with its projection under the forget-one-coordinate map and use Lebesgue measure on the image).
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