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Lattice Points on Hyperboloids of One Sheet New York Journal of Mathematics New York J. Math. 20 (2014) 1253{1268. Lattice points on hyperboloids of one sheet Arthur Baragar Abstract. The problem of counting lattice points on a hyperboloid of two sheets is Gauss' circle problem in hyperbolic geometry. The problem of counting lattice points on a hyperboloid of one sheet does not have the same geometric interpretation, and in general, the solution(s) to Gauss' circle problem gives a lower bound, but not an upper bound. In this paper, we describe an exception. Given an ample height, and a lattice on a hyperboloid of one sheet generated by a point in the interior of the effective cone, the problem can be reduced to Gauss' circle problem. Contents Introduction 1253 1. The Gauss circle problem 1255 1.1. Euclidean case 1255 1.2. Hyperbolic case 1255 2. An instructive example 1256 3. The main result 1260 4. Motivation 1264 Appendix: The pseudosphere in Lorentz space 1265 References 1267 Introduction Let J be an (m + 1) × (m + 1) real symmetric matrix with m ≥ 2 and signature (m; 1) (that is, J has m positive eigenvalues and one negative t m+1 eigenvalue). Then x ◦ y := x Jy is a Lorentz product and R equipped m;1 with this product is a Lorentz space, denoted R . The hypersurface m;1 Vk = fx 2 R : x ◦ x = kg Received February 26, 2013; revised December 9, 2014. 2010 Mathematics Subject Classification. 11D45, 11P21, 20H10, 22E40, 11N45, 14J28, 11G50, 11H06. Key words and phrases. Gauss' circle problem, lattice points, orbits, Hausdorff dimen- sion, ample cone. This work is based upon research supported by the National Security Agency under grant H98230-08-1-0022. ISSN 1076-9803/2014 1253 1254 ARTHUR BARAGAR is a hyperboloid of two sheets if k < 0, and a hyperboloid of one sheet if k > 0. For k < 0, let us pick one of the sheets and denote it with H. Then H =∼ m H has a natural hyperbolic structure, where the distance jABj between two points on H is defined via the equation k cosh jABj = A ◦ B. (See the appendix for a short synopsis of this model of hyperbolic geometry.) Let t O = O(R) = fT 2 M(m+1)×(m+1)(R): T JT = Jg and + + O = O (R) = fT 2 O : T H = Hg: Then O+ is the group of isometries on H. Let Γ be a discrete subgroup of O+. That is, for any P 2 H, there exists a real M (that may depend on P ) such that jPQj > M for all Q 6= P in the Γ-orbit of P . Suppose further that Γ has the finite geometric property, which is to say Γ, as a group acting on H, has a polyhedral fundamental domain F ⊂ H with a finite number of faces. Let us define the quantity N(P; D; Γ; t) = #fγ 2 Γ: jγP ◦ Dj < tg; m;1 for P and D in R . If there exist P and D on H such that N(P; D; Γ; t) m − 1 lim sup > ; t!1 log t 2 then we say Γ is sufficiently thick. In that case, the limit N(P; D; Γ; t) lim t!1 log t exists and depends only on Γ (see Theorem 1.1 below due to Lax and Phillips). Let us call this limit α(Γ). It has a variety of interpretations, which are discussed following Theorem 1.1. Our main result is to extend this classical result to include certain cases m;1 when P is on a hyperboloid of one sheet. Let us call K ⊂ R an ample cone if K is convex, open, is a cone with vertex 0 (that is, λx 2 K for all x 2 K and λ > 0), is fixed by Γ (that is, γx 2 K for all x 2 K and γ 2 Γ), and x ◦ x < 0 for all x 2 K. Given an ample cone K, let us define the effective cone to be m;1 E = fx 2 R : x ◦ y < 0 for all y 2 Kg: Note that E is closed. We say x is ample if x 2 K, and effective if x 2 E. The terms are borrowed from algebraic/arithmetic geometry, wherein a problem motivated discovery of our main result, which is the following: Theorem 0.1. Let Γ be a discrete group of isometries with the finite geo- metric property, and let K be an ample cone with respect to Γ. Suppose that D is ample and P lies in the interior of the effective cone. If Γ is sufficiently thick, then log N(P; D; Γ; t) lim = α(Γ): t!1 log t LATTICE POINTS ON HYPERBOLOIDS OF ONE SHEET 1255 If Γ is not sufficiently thick, then log N(P; D; Γ; t) m − 1 lim sup ≤ : t!1 log t 2 Our motivation is discussed in Section4. Those who are already familiar with the ample and effective cone might have noticed the peculiar direction of the inequality in the definition of E. This is because we chose J to have signature (m; 1), rather than the signature (1; m) of the intersection pairing. 1. The Gauss circle problem 1.1. Euclidean case. How many integer lattice points are there in a circle of radius r? More precisely, what is 2 n(r) := #fQ 2 Z : jOQj < rg; where O = (0; 0)? By thinking of each lattice point as the center of a square tile of unitp area, we see that n(r) is bounded below by the areap of a disc of radius r − 2 and above by the area of a disc with radius r + 2. Thus, n(r) = πr2 + E(r); p where jE(r)j < 2π(r 2 + 1). The first nontrivial bound on the error term is due to Sierpinski (1906), who showed jE(r)j = O(r2=3); the current best known bound O(r46=73+) is due to Huxley [Hux93]; and the conjectured best possible bound is O(r1=2+) (where the constants implied by the big O depend on > 0). The lattice can be thought of as the orbit of the point O under the action of a group Γ, and a tile can be thought of as a fundamental domain F. In higher dimensions, we get the elementary asymptotic for n(r) of the volume of the ball B(r) of radius r, divided by the volume of the fundamental domain F, with an error term no larger than a constant multiple of the surface area of the ball. That is, jB(r)j n(r) = + O(j@B(r)j): jFj 1.2. Hyperbolic case. In hyperbolic geometry, we have analogous results, but no geometric proofs. Given a discrete group of isometries Γ with a fundamental domain F (with natural properties), and points P and O in m H , we have jB(r)j (1) #fσ 2 Γ: jσ(P )Oj < rg = + o(jB(r)j): jFj Let us call this quantity n(P; O; Γ; r). For P inside F, this counts the number of points in the Γ-orbit of P . For P on the boundary of F, it 2 is out by a factor of j Stab(P )j. The area of a disc of radius r in H is jB(r)j = 2π(cosh r − 1) ∼ πer, so j@B(r)j = O(jB(r)j) (take the derivative), 2 which explains why there are no geometric proofs. For compact F in H , 1256 ARTHUR BARAGAR the result in Equation (1) was proved by Huber [Hub56]. It was generalized to F with finite area by Patterson [Pat75], and the generalization to m dimensions is attributed to Selberg (see [LP82]). Unlike in Euclidean geometry, the hyperbolic case when F has infinite volume is very interesting. In particular, we have the following result referred to earlier, which is a simplification of results due to Lax and Phillips [LP82, Theorems 1 and 5.7]. Their results are worded in terms of the eigenvalues for the Laplace{Beltrami operator associated to Γ. Theorem 1.1 (Lax and Phillips, [LP82]). Suppose Γ is a group of isometries m on H that has the finite geometric property. If the discrete part of the spectrum for the associated Laplace{Beltrami operator is not empty, then log n(P; O; Γ; r) lim r!1 r m−1 exists, depends only on Γ, and is greater than 2 . If the discrete part of the spectrum is empty, then log n(P; O; Γ; r) m − 1 lim sup ≤ : r!1 r 2 For P and D on H, the quantities N and n are related by (2) N(P; D; Γ; t) = n(P; D; Γ; cosh−1(−t=k)): Thus, Γ is sufficiently thick if and only if the discrete part of the spec- trum is not empty. The discrete part of the spectrum lies in the interval m−1 2 2 −( 2 ) ; 0 , and α(Γ) is the largest root of x − (m − 1)x + λ0 = 0, where λ0 is the largest eigenvalue in the spectrum. The quantity α(Γ) can also be interpreted as the Hausdorff dimension of m m−1 m the limit set Λ(Γ) for Γ [Sul82]. Fix P in H and let S = @H be the m usual compactification of H . The limit set Λ(Γ) is the set of points y on m−1 m m S = @H such that for any hyperplane in H that does not contain y, there exists an x 2 Γ(P ) such that x and y are on the same side of the m hyperplane.
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