NORMAL ZETA FUNCTIONS OF THE HEISENBERG GROUPS OVER NUMBER RINGS I – THE UNRAMIFIED CASE
MICHAEL M. SCHEIN AND CHRISTOPHER VOLL
Abstract. Let K be a number field with ring of integers OK . We compute the
local factors of the normal zeta functions of the Heisenberg groups H(OK ) at rational primes which are unramified in K. These factors are expressed as sums, indexed by Dyck words, of functions defined in terms of combinatorial objects such as weak orderings. We show that these local zeta functions satisfy functional equations upon the inversion of the prime.
1. Introduction 1.1. Normal zeta functions of groups. If G is a finitely generated group, then the C numbers an (G) of normal subgroups of G of index n in G are finite for all n ∈ N. In their seminal paper [7], Grunewald, Segal, and Smith defined the normal zeta function of G to be the Dirichlet generating function
∞ C X C −s ζG(s) = an (G)n . n=1 Here s is a complex variable. If G is a finitely generated nilpotent group, then its normal zeta function converges absolutely on some complex half-plane. In this case the Euler product decomposition C Y C ζG(s) = ζG,p(s) p prime holds, where the product runs over all rational primes, and for each prime p,
∞ C X C −ks ζG,p(s) = apk (G)p k=0 counts normal subgroups of G of p-power index in G; cf. [7, Proposition 4]. The Euler C −s factors ζG,p(s) are all rational functions in p ; cf. [7, Theorem 1].
Date: September 5, 2014. 2010 Mathematics Subject Classification. 20E07, 11M41, 05A10, 05A15, 20F18. Key words and phrases. Finitely generated nilpotent groups, normal zeta functions, Dyck words, generating functions, functional equations. Schein was supported by grant 2264/2010 from the Germany-Israel Foundation for Scientific Research and Development and a grant from the Pollack Family Foundation. We acknowledge support by the DFG Sonderforschungsbereich 701 “Spectral Structures and Topological Methods in Mathematics” at Bielefeld University. 2 MICHAEL M. SCHEIN AND CHRISTOPHER VOLL
For any ring R the Heisenberg group over R is defined as 1 a c (1.1) H(R) = 0 1 b | a, b, c ∈ R . 0 0 1
In this paper we study the normal zeta functions of the Heisenberg groups H(OK ), where OK is the ring of integers of a number field K. The groups H(OK ) are finitely generated, nilpotent of class two, and torsion-free. g Let n = [K : Q] and g ∈ N. Given g-tuples e = (e1, . . . , eg) ∈ N and f = g Pg (f1, . . . , fg) ∈ N satisfying i=1 eifi = n, we say that a (rational) prime p is of decom- position type (e, f) in K if e1 eg pOK = p1 ··· pg , where the pi are distinct prime ideals in OK with ramification indices ei and inertia degrees fi = [OK /pi : Fp] for i = 1, . . . , g. Note that this notion of decomposition type features some redundancy reflecting the absence of a natural ordering of the prime ideals of OK lying above p. One of the main results of [7] asserts that the Euler factors ζ/ (s) are rational in the two parameters p−s and p on sets of primes of fixed H(OK ),p decomposition type in K:
g g Pg Theorem 1.1. [7, Theorem 3] Given (e, f) ∈ N × N with i=1 eifi = n, there exists C a rational function We,f (X,Y ) ∈ Q(X,Y ) such that, for all number fields K of degree [K : Q] = n and for all primes p of decomposition type (e, f) in K, the following identity holds: ζC (s) = W C (p, p−s). H(OK ),p e,f g We write 1 for the vector (1,..., 1) ∈ N , all of whose components are ones. We g remark (see (1.3)) that if H(Z) denotes the direct product of g copies of H(Z), then for all primes p we have −s / W C (p, p ) = ζ g (s). 1,1 H(Z) ,p C 1.2. Main results. In Theorem 3.6 we explicitly compute the functions W1,f (X,Y ), thereby finding the Euler factors ζC at all rational primes p that are unramified H(OK ),p C in K, i.e. those for which e = 1. The functions W1,f (X,Y ) are expressed as sums, indexed by Dyck words, where each summand is a product of functions that can be in- terpreted combinatorially. We use the explicit formulae to prove the following functional equations:
g Pg Theorem 1.2. Let f ∈ N with i=1 fi = n. Then 3n C −1 −1 3n ( ) 5n C (1.2) W1,f (X ,Y ) = (−1) X 2 Y W1,f (X,Y ). By [18, Theorem C], the Euler factors ζ/ satisfy a functional equation upon H(OK ),p inversion of the parameter p for all but finitely many p. However, the methods of that paper do not determine the finite set of exceptional primes. In general it is not known whether any functional equation obtains at the exceptional primes. For the Heisenberg groups, we establish such functional equations for non-split primes in the forthcoming paper [10]: NORMAL ZETA FUNCTIONS OF HEISENBERG GROUPS OVER NUMBER RINGS I 3
Theorem 1.3. [10, Theorem 1.1] Let e, f ∈ N with ef = n. Then 3n C −1 −1 3n ( 2 ) 5n+2(e−1)f C W(e),(f)(X ,Y ) = (−1) X Y W(e),(f)(X,Y ). Based on Theorems 1.2 and 1.3 and computations of Euler factors that we have performed in other cases for n = 4, we conjecture the existence of a functional equation at all primes for Heisenberg groups over number rings.
g g Pg Conjecture 1.4. Let (e, f) ∈ N × N with i=1 eifi = n. Then 3n Pg C −1 −1 3n ( ) 5n+ i=1 2(ei−1)fi C We,f (X ,Y ) = (−1) X 2 Y We,f (X,Y ).
In particular we conjecture that, for the groups H(OK ), the finite set of rational primes excluded in [18, Theorem C] consists precisely of the primes that ramify in K. The conjectured existence of a functional equation at all primes is remarkable, since in general this does not hold even for groups where a functional equation is satisfied at all but finitely many primes by [18, Theorem C]. C Our methods in fact allow the rational functions We,f (X,Y ) to be determined explic- itly for any decomposition type (e, f). However, if g > 1 and e 6= 1, then we do not in general know how to interpret these explicit formulae in terms of functions that are known to satisfy a functional equation. Conjecture 1.4 has been verified for all cases occurring for n ≤ 4. Prior to this work, the functions ζ/ had been known only in a very limited H(OK ),p number of cases; see [5, Section 2] for a summary of the previously available results. In [7, Section 8] the local functions were computed for all primes when n = 2 and for the inert and totally ramified primes when n = 3. The remaining cases for n = 3 were computed in Taylor’s thesis [16], using computer-assisted calculations of cone integrals; C see [4]. Finally, Woodward determined W1,1(X,Y ) for n = 4. The numerator of this rational function is the first polynomial in [5, Appendix A], where it takes up nearly a full page. Example 5.2 below exhibits how our method produces this function as a sum of fourteen well-understood summands.
1.3. Related work and open problems. In the recent past, zeta functions associated to Heisenberg groups and their various generalizations have often served as a test case for an ensuing general theory. For instance, the seminal paper [7] contains special cases of the computations done in the present paper as examples. Similarly, Ezzat [6] computed the representation zeta functions of the groups H(OK ) for quadratic number rings OK , enumerating irreducible finite-dimensional complex representations of such groups up to twists by one-dimensional representations. The paper [15] develops a general framework for the study of representation zeta functions of finitely generated nilpotent groups. Moreover, it generalized Ezzat’s explicit formulae to arbitrary number rings and more general group schemes. The current paper leaves open a number of challenges. One of them is the computation / g of the rational functions We,f for general e ∈ N ; in the special case g = 1, this has been achieved in [10]. Another one is the computation of the local factors of the subgroup zeta function ζH(OK )(s) enumerating all subgroups of finite index in H(OK ). This has not even been fully achieved for quadratic number rings OK . 4 MICHAEL M. SCHEIN AND CHRISTOPHER VOLL
More generally, it is of interest to compute the (normal) subgroup zeta functions of other finitely generated nilpotent groups, and their behavior under base extension. We refer to [5] for a comprehensive list of examples. In his M.Sc. thesis [1], Bauer has generalized many of our results to the normal zeta functions of the higher Heisenberg groups Hm(OK ) for all m ∈ N, where Hm is a centrally amalgamated product of m Heisenberg groups. In other words, if R is a ring and we view elements of Rm as row vectors, and if Im denotes the m × m identity matrix, then 1 a c T m Hm(R) = 0 Im b | a, b ∈ R , c ∈ R . 0 0 1 The paper [8] arose from the (uncompleted) project to compute the subgroup zeta functions ζHm(Z)(s). 1.4. Structure of the proofs of the main results. The problem of counting normal subgroups in a finitely generated torsion-free nilpotent group of nilpotency class 2 is known to be equivalent to that of counting ideals in a suitable Lie ring; cf. [7, Section 4]. Specifically, let Z be the center of H(OK ); it is easy to see that this is the subgroup of matrices satisfying a = b = 0 in the notation of (1.1), and that it coincides with the derived subgroup of H(OK ). Define the Lie ring
L = Z ⊕ (H(OK )/Z) , with Lie bracket induced by commutators in the group H(OK ). It is easy to verify that ∼ L = L(OK ) where, more generally and in analogy with (1.1), the Heisenberg Lie ring over an arbitrary ring R is defined as 0 a c L(R) = 0 0 b | a, b, c ∈ R , 0 0 0 with Lie bracket induced from gl3(R). The ideal zeta function of L(OK ) is the Dirichlet generating function ∞ X ζ/ (s) = aC(L(O ))n−s, L(OK ) n K n=1 C where an (L(OK )) denotes the number of ideals of L(OK ) of index n in L(OK ). This zeta function, too, satisfies an Euler product decomposition, of the form ∞ Y X ζ/ (s) = ζ/ (s) = aC (L(O ))p−ks. L(OK ) L(OK ),p pk K p prime k=0 By the remark following [7, Lemma 4.9] we have, for all primes p, that ζC = ζ/ . H(OK ),p L(OK ),p 0 Now set Rp = OK ⊗Z Zp and Lp = L(Rp) for every prime p. We write Lp = [Lp,Lp] for the derived subring and center of Lp, and denote by Lp the abelianization Lp/[Lp,Lp]. 0 The Zp-modules underlying Lp and Lp have ranks n and 2n, respectively. Then ∼ 0 Lp = L(Rp) = Lp ⊕ Lp. NORMAL ZETA FUNCTIONS OF HEISENBERG GROUPS OVER NUMBER RINGS I 5
/ / The Euler factor ζ may be identified with the ideal zeta function ζ of the p-Lie L(OK ),p Lp Z lattice Lp, enumerating Zp-ideals of Lp of finite additive index in Lp. To summarize, the following equalities hold for all primes p:
(1.3) ζ/ = ζ/ = ζ/ = ζ/ . H(OK ),p L(OK ),p L(Rp) Lp Essentially by [7, Lemma 6.1], we have that
/ X −s X 0 2n−s (1.4) ζLp (s) = |Lp : Λ| |Lp : M| . 0 Λ≤f Lp [Λ,Lp]≤M≤Lp
Here the outer sum runs over all Zp-sublattices Λ ≤ Lp of finite additive index. We briefly summarize our strategy for computing the right hand side of (1.4). Let p be a prime of decomposition type (e, f) in K. In Lemma 2.2 we determine the isomorphism 0 type of the finite p-group Lp/[Λ,Lp] for every finite-index sublattice Λ ≤f Lp. More n precisely, we associate to Λ an n-tuple ` = `(Λ) = (`1, . . . , `n) ∈ N0 such that
0 `1 `n Lp/[Λ,Lp] ' Z/p Z × · · · × Z/p Z. Noting that the inner sum of (1.4) depends only on ` and not on Λ, we proceed to evaluate the outer sum in terms of the parameters `; cf. Lemma 2.4. By this point, we are able to transform (1.4) into the equation
g ! / Y −2fis C e,f −s ζ (s) = (1 − p ) ζ 2n (s) D (p, p ), Lp Zp i=1 where
e,f −s X −2s Pn ` X (2n−s)Pn µ (1.5) D (p, p ) = p i=1 i α(λ(`), µ; p) p i=1 i ;
`∈Adme,f µ≤λ(`)
C cf. Lemma 2.19. The zeta function ζ 2n (s) is well-known; cf. (2.9). We now explain the Zp meanings of the terms in (1.5). n The set Adme,f ⊆ N0 of admissible n-tuples only depends on the decomposition type n (e, f) of p in K; cf. Definition 2.3. For an n-tuple ` ∈ N0 , we define λ(`) to be the partition λ1 ≥ · · · ≥ λn obtained by arranging the components of ` in non-ascending order. As ` runs over Adme,f , the partitions λ(`) run over all the possible elementary 0 divisor types of commutator lattices [Λ,Lp] ≤ Lp. The inner sum on the right hand side of (1.5) runs over all partitions µ which are dominated by λ(`). Finally, α(λ(`), µ; p) denotes the number of abelian p-groups of type µ contained in a fixed abelian p-group of type λ(`). A classical formula of Birkhoff expresses this number in terms of the dual partitions of λ(`) and µ; see Proposition 2.15. So far, everything we have said holds for all decomposition types (e, f). The difficulty in evaluating (1.5) comes from the strong dependence of α(λ(`), µ; p) on the relative sizes of the parts of the partitions λ(`) and µ. For unramified primes, we overcome this difficulty by splitting D1,f into a finite sum of more tractable functions. Indeed, the different ways in which the partition λ(`) can “overlap” the partition µ are parametrized by Dyck words of length 2n; see Section 2.4 for details. Given such a Dyck word w, we 6 MICHAEL M. SCHEIN AND CHRISTOPHER VOLL
1,f 1,f define a sub-sum Dw of D running over pairs of partitions (λ(`), µ) whose overlap is captured by w, so that 1,f X 1,f D = Dw , w∈D2n where D2n is the set of Dyck words of length 2n; see Section 2.6. The cardinality of D2n 1 2n is the n-th Catalan number Catn = n+1 n . 1,f Each Dw can be expressed in terms of the Igusa functions introduced in [17] and their partial generalizations defined in Section 2.3. The latter may be interpreted as fine Hilbert series of Stanley-Reisner rings of barycentric subdivisions of simplices. Stanley proved that these rational functions satisfy a functional equation upon inversion of 1,f their variables. We deduce that the functions Dw all satisfy a functional equation whose symmetry factor is independent of the Dyck word w. This allows us to prove Theorem 1.2.
Acknowledgements. We are grateful to Mark Berman for bringing us together to work on this project, to Kai-Uwe Bux for conversations about face complexes, and to the referee for helpful comments.
2. Preliminaries
Throughout this paper, K is a number field of degree n = [K : Q] with ring of −s integers OK . By p we denote a rational prime, and we fix the abbreviation t = p . For an integer m ≥ 1, we write [m] for {1, 2, . . . , m} and [m]0 for {0, 1, . . . , m}. Given integers a, b with a ≤ b, we write [a, b] for {a, a+1, . . . , b}, and ]a, b] for {a+1, a+2, . . . , b}.
g g 2.1. Lattices. Suppose that p has decomposition type (e, f) ∈ N ×N in K, in the sense e1 eg defined in Section 1.1. Then p decomposes in K as pOK = p1 ··· pg , where p1,..., pg are distinct prime ideals in OK . For each i ∈ [g], let ki = OK /pi be the corresponding Pi residue field. Then fi = [ki : Fp]. We define Ci = j=1 ejfj for each i ∈ [g]0, so that 0 = C0 < C1 < ··· < Cg = n.
Let Rp = OK ⊗Z Zp. This ring is a free Zp-module of rank n. It splits into a direct (1) (g) (i) product Rp = Rp ×· · ·×Rp , where for each i ∈ [g] the component Rp is just the local (i) (i) (i) ring OK,pi . For each i ∈ [g], we choose a uniformizer πi ∈ Rp , an Fp-basis {β1 ,..., βfi } (i) (i) (i) of ki, and a lift βj ∈ Rp of βj ∈ ki for each j ∈ [fi]. Then the set
n (i) s o Bi := βj πi | j ∈ [fi], s ∈ [ei − 1]0
(i) is a basis of Rp as a Zp-module; see, for instance, the proof of [9, Proposition II.6.8]. The union of the bases Bi, for i ∈ [g], constitutes a basis {α1, . . . , αn} of Rp as a Zp-module. We index it as follows: (i) s βj πi = αCi−1+sfi+j. km We define structure constants cu ∈ Zp, for k, m, u ∈ [n], with respect to this basis, via n X km (2.1) αkαm = cu αu. u=1 NORMAL ZETA FUNCTIONS OF HEISENBERG GROUPS OVER NUMBER RINGS I 7
km Note that cu = 0 unless there exists an i ∈ [g] such that k, m ∈ ]Ci−1,Ci]. Hence we obtain the following presentation of the Zp-Lie ring Lp = H(Rp):
* n + X km Lp = x1, . . . , xn, y1, . . . , yn, z1, . . . , zn | [xk, ym] = cu zu, for k, m ∈ [n] . u=1 Here it is understood that all unspecified Lie brackets vanish. It is clear that the cen- ter of this Lie ring, which is equal to the derived subring, is spanned by {z1, . . . , zn}. Similarly, the abelianization Lp = Lp/[Lp,Lp] is spanned by the images of the ele- ments x1, . . . , xn, y1, . . . , yn. We abuse notation and denote these elements of Lp by x1, . . . , xn, y1, . . . , yn as well. Let Λ ≤ Lp be a sublattice of finite index. Then Λ is a free Zp-module of rank 2n. Let (b1, . . . , b2n) be an ordered Zp-basis for Λ. Observe that each bj can be expressed uniquely in the form n n X X (2.2) bj = b2k−1,jxk + b2k,jyk. k=1 k=1 for some b1,j, . . . , b2n,j ∈ Zp. We set B(Λ) = (bk,j) ∈ Mat2n(Zp). Conversely, the columns of any matrix B ∈ Mat2n(Zp) with det B 6= 0 encode generators of a sublattice of Lp of finite index in Lp, by means of (2.2). The matrix B(Λ) depends on the choice of basis; indeed, two matrices B,B0 represent the same lattice if and only if there exists 0 some A ∈ GL2n(Zp) such that B = BA. If F/Qp is a finite extension, we denote by valF the normalized valuation on F . We simply write val instead of valQp . For each i ∈ [g] we define the following two parameters:
(2.3) εi(Λ) = min {val(bk,j) | k ∈ ]2Ci−1, 2Ci], j ∈ [2n]} ,
δi(Λ) = min {d ∈ [ei − 1]0 |
(2.4) ∃k ∈ ]2Ci−1 + 2dfi, 2Ci−1 + 2(d + 1)fi], j ∈ [2n] : val(bk,j) = εi(Λ) .
Informally, εi(Λ) is the smallest valuation of any element appearing on or between the (2Ci−1 + 1)-st and (2Ci)-th rows of the matrix B(Λ). If we split this range of 2eifi rows into ei blocks of 2fi consecutive rows each, then δi(Λ) is the largest number such that the first δi(Λ) blocks contain no matrix elements of minimal valuation εi(Λ). It is easy to see that εi(Λ) and δi(Λ) are independent of the choice of basis and so are well-defined. Definition 2.1. For j ∈ [n] we set ( εi(Λ) + 1, if j ∈ ]Ci−1,Ci−1 + δi(Λ)fi], `j = εi(Λ), if j ∈ ]Ci−1 + δi(Λ)fi,Ci].
n and set `(Λ) = (`1, . . . , `n) ∈ N0 .
Informally, the n-tuple `(Λ) is a concatenation of g blocks of lengths e1f1, . . . , egfg. Within each block, the components are all equal, except that for each i ∈ [g] the first δi(Λ)fi components of the i-th block are incremented by 1. Thus `(Λ) just depends on the ramification type (e, f) and the parameters εi(Λ), δi(Λ) for each i ∈ [g]. 8 MICHAEL M. SCHEIN AND CHRISTOPHER VOLL
Lemma 2.2. Let Λ ≤ Lp be a sublattice of finite index, and let `(Λ) be as in Defini- tion 2.1. Then n 0 ∼ Y `j Lp/[Λ,Lp] = Z/p Z. j=1 Proof. It is clear that (2.5) [Λ,L ] = span {[b , x ], [b , y ] | j ∈ [2n], k ∈ [n]} . p Zp j k j k
For each i ∈ [g], we define the following sublattice of [Λ,Lp]: [Λ,L ] = span {[b , x ], [b , y ] | j ∈ [2n], k ∈ ]C ,C ]} . p i Zp j k j k i−1 i Lg 0 By the observation following (2.1), [Λ,Lp] = i=1[Λ,Lp]i. Moreover, if we set Lp(i) to 0 be the Zp-submodule of Lp generated by {zCi−1+1, . . . , zCi }, then it is clear that g 0 Y 0 Lp/[Λ,Lp] ' Lp(i)/[Λ,Lp]i. i=1 We have thus reduced to the case where p is non-split in K, i.e. g = 1. So suppose e that pOK = p is non-split in K and write ε, δ for ε1(Λ), δ1(Λ) as in (2.3), (2.4). Then Rp is a local ring with residue field k ' Fpf , where ef = n. Let π ∈ Rp be a uniformizer.
Let F be the fraction field of Rp, and note that (valF )|Qp = e · val. As before, we choose s a Zp-basis (α1, . . . , αn) of the form αsf+j = βjπ , where j ∈ [f] and s ∈ [e − 1]0, and the image in k of {β1, . . . , βf } is an Fp-basis of k. Let Λ be given by a matrix B(Λ) ∈ Mat2n(Zp) as above. Then ε is just the minimal valuation attained by the entries of B(Λ). To prove the lemma, it suffices to establish the following claim.
2n Claim. Let (v1, . . . , v2n) ∈ Zp . Set 0 00 ε = min{val(v2k−1) | k ∈ [n]}, ε = min{val(v2k) | k ∈ [n]} and define 0 0 δ = min{d ∈ [e − 1]0 | ∃k ∈ ]df, (d + 1)f] : val(v2k−1) = ε }, 00 00 δ = min{d ∈ [e − 1]0 | ∃k ∈ ]df, (d + 1)f] : val(v2k) = ε }. Pn Consider the element v = k=1(v2k−1xk + v2kyk) ∈ Λ. Then (2.6) ε0+1 ε0+1 ε0 ε0 span {[v, y ],..., [v, y ]} = span {p z , . . . , p z 0 , p z 0 , . . . , p z }, Zp 1 n Zp 1 δ f δ f+1 n ε00+1 ε00+1 ε00 ε00 span {[v, x ],..., [v, x ]} = span {p z , . . . , p z 00 , p z 00 , . . . , p z }. Zp 1 n Zp 1 δ f δ f+1 n Indeed, assuming the claim, it easily follows from (2.5) that [Λ,L] = span {pε+1z , . . . , pε+1z , pεz , . . . , pεz }. Zp 1 δf δf+1 n Now we prove the claim. We only consider the statement involving ε0 and δ0, since the other half of the claim is dealt with analogously. It is clear that neither side of (2.6) 0 Pn 0 −ε0 0 changes if we replace v by v = k=1 v2k−1xk. Moreover, replacing v with p v we may assume without loss of generality that ε0 = 0. NORMAL ZETA FUNCTIONS OF HEISENBERG GROUPS OVER NUMBER RINGS I 9
Now let l ∈ [n] be the smallest number such that val(v2l−1) = 0. Then l satisfies l ∈ ]δ0f, (δ0 + 1)f] by the definition of δ0. Observe that for each m ∈ [n] we have, by (2.1), " n # n n ! 0 X X X km (2.7) [v , ym] = v2k−1xk, ym = v2k−1cu zu. k=1 u=1 k=1
It follows from our definition of the basis (α1, . . . , αn) that valF (αk) = d if k ∈ 0 0 0 ]df, (d + 1)f]. In particular, if k > δ f, then valF (αk) ≥ δ and hence valF (αkαm) ≥ δ km 0 for all m. Since the αk are linearly independent over Zp, it follows that valF (cu αu) ≥ δ 0 0 km for all u ∈ [n]. Thus, if k > δ f but u ≤ δ f, then we must have valF (cu ) > 0 and km 0 hence val(cu ) > 0. On the other hand, if k ≤ δ f then val(v2k−1) > 0 by the definition 0 0 Pn km of δ . Therefore, if u ≤ δ f, then val k=1 v2k−1cu ≥ 1. It follows by (2.7) that the left hand side of (2.6) is contained in the right hand side. 0 0 Let M = (Mum) ∈ Matn(Zp) be the matrix whose columns are [v , y1],..., [v , yn], 0 Pn km with respect to the basis (z1, . . . , zn) of Lp. Then Mum = k=1 v2k−1cu , and it follows from the definition of the structure constants that M is the matrix of the Zp-linear operator n ! X Rp → Rp, x 7→ v2k−1αk x k=1 Pn with respect to the basis (α1, . . . , αn) of Rp. Hence det M = NF/Qp ( k=1 v2k−1αk), where NF/Qp denotes the norm function. By the considerations in the previous paragraph we see that all the entries in the first δ0f rows of M are divisible by p. 0 Let ∆δ0f ∈ GLn(Qp) be the diagonal matrix such that the first δ f diagonal entries are −1 0 0 p and the remaining diagonal entries are 1. Let M = ∆δ0f M. Then M ∈ Mn(Zp). Pn 0 0 0 As valF ( k=1 v2k−1αk) = δ , it follows that val(det M ) = val(det M) − δ f = 0. Thus 0 0 the matrix M is invertible, and the space spanned by its columns is just Lp. It follows that pz1, . . . , pzδ0f are contained in the span of the columns of M. Hence the right hand side of (2.6) is contained in the left hand side. This completes the proof of the claim. g g n Definition 2.3. Let (e, f) ∈ N × N . We say that an n-tuple ` = (`1, . . . , `n) ∈ N0 is admissible for (e, f) if there exists a sublattice Λ ≤ Lp of finite index such that `(Λ) = `. This is equivalent to the condition that for, each i ∈ [g], there exist δi ∈ [ei − 1]0 such that
(2.8) `Ci−1+1 = ··· = `Ci−1+δifi = `Ci−1+δifi+1 + 1 = ··· = `Ci + 1. n We denote the set of admissible n-tuples by Adme,f ⊆ N0 . We sometimes make use of the fact that an admissible n-tuple ` determines, and is determined by, the pair of g-tuples ((`C1 , . . . , `Cg ), (δ1, . . . , δg)) in (2.8). Note that n Adm1,1 = N0 . The opposite extreme occurs for (e, f) = ((1), (n)), where Adm(1),(n) = 1N0 consists of n-tuples all of whose components are equal. Recall that, for d ∈ N, d−1 1 C Y (2.9) ζ d (s) = ζp(s − i) = , Zp Qd−1 i−s i=0 i=0 (1 − p ) 10 MICHAEL M. SCHEIN AND CHRISTOPHER VOLL
−s −1 where ζp(s) = (1 − p ) is the local Riemann zeta function; cf., for instance, [7, Proposition 1.1]. Since Lp is a free abelian Lie ring of rank 2n over Zp, we have that C C ζ (s) = ζ 2n (s). Lp Zp
Lemma 2.4. Let p be a prime of decomposition type (e, f) in K. Given an n-tuple ` = (`1, . . . , `n) ∈ Adme,f , we have