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52  Hyperbolic Coxeter polytopes and finite covolume

Remark 52.1 (Orientation) Vinberg’s algorithm (Theorem 49.18) returns a set of simple roots and terminates when they bound a chamber of finite volume. The condition it tests is a condition on the vertices of that chamber, and those vertices are read off the Coxeter–Vinberg diagram.

Throughout, \(L\) is a hyperbolic lattice of signature \((1,n)\), \(C_L^+\) is a fixed component of its positive cone, and \(\HH^n_L\) is the associated hyperbolic \(n\)-space of Definition 49.9. Roots are negative-definite directions, so a mirror \(H_v = v^{\perp}\intersect\HH^n_L\) is nonempty exactly when \(v^2 < 0\).

52.1 The Tits cone

Definition 52.2 Let \((W,S)\) be a Coxeter system with Gram form \(G\) on \(V \definedas \RR^S\) and simple roots \(\theset{\alpha_s}_{s\in S}\), and let \(W\) act on the dual space \(V^*\) by the contragredient of its action on \(V\). The closed fundamental chamber is \[ \overline{C} \definedas \theset{\, f\in V^* \mid f(\alpha_s)\geq 0 \text{ for all } s\in S \,} , \] and the Tits cone of \((W,S)\) is \[ U \definedas \Union_{w\in W} w\!\left(\overline{C}\right) \iscontainedin V^* . \] When \(G\) is nondegenerate it identifies \(V\) with \(V^*\), and \(\overline C\) becomes the cone \(\theset{v\in V \mid \beta(v,\alpha_s)\geq 0}\) over the closure of a Weyl chamber in the sense of Definition 49.13.

Theorem 52.3 (Properties of the Tits cone) Let \((W,S)\), \(\overline C\) and \(U\) be as in Definition 52.2, and for \(f\in \overline C\) write \(I(f) \definedas \theset{s\in S \mid f(\alpha_s) = 0}\).

  1. \(U\) is a convex cone, and \(\overline{C}\) is a strict fundamental domain for the action of \(W\) on \(U\): every \(W\)-orbit in \(U\) meets \(\overline{C}\) in exactly one point.

  2. The stabilizer of \(f\in\overline{C}\) in \(W\) is the standard parabolic subgroup \(W_{I(f)}\) of Definition 50.12.

  3. A point \(f\in\overline{C}\) lies in the interior of \(U\) if and only if \(W_{I(f)}\) is finite, that is, if and only if the subdiagram \(\Sigma_{I(f)}\) is elliptic.

  4. \(W\) acts properly discontinuously on the interior of \(U\).

These are theorems of Tits, recorded in (Bourbaki 2008; Humphreys 1990).

Remark 52.4 For a spherical Coxeter system \(U = V^*\). For an irreducible euclidean one, with \(\ker G = \RR\delta\) as in Proposition 50.20, \(U\) is the open half-space \(\theset{f \mid f(\delta) > 0}\) together with the origin, and the affine reflection group of Theorem 50.10(2) is its action on the hyperplane \(f(\delta) = 1\).

For a hyperbolic Coxeter system, part (3) says which points of the chamber have finite stabilizer, and Proposition 52.6 identifies those on the boundary of the cone: an ordinary vertex of the polytope lies in the interior of \(U\), and an ideal vertex does not, its stabilizer being an infinite euclidean reflection group.

52.2 Vertices of a Coxeter polytope

Notation 52.5 Let \(P\iscontainedin\HH^n_L\) be a Coxeter polytope (Definition 49.11) with walls \(H_s = \alpha_s^{\perp}\) indexed by a finite set \(S\), and Coxeter–Vinberg diagram \(\Sigma\). For \(I\iscontainedin S\) write \[ A_I \definedas \spanof_\RR\theset{\, \alpha_s \mid s\in I \,} \iscontainedin L_\RR . \] The Gram matrix \(G[I,I]\) is the Gram matrix of a spanning set of \(A_I\), so its rank equals the rank of \(\restrictionof{\beta_L}{A_I}\); that common value is the rank of \(\Sigma_I\) in the sense of Definition 50.12.

Proposition 52.6 (Vertices and subdiagrams)

Let \(I\iscontainedin S\).

  1. If \(\Sigma_I\) is elliptic of rank \(n\), then \(\dim A_I = n\), the orthogonal complement \(A_I^{\perp L_\RR}\) is a line spanned by a vector of positive norm, and \[ \Intersect_{s\in I} H_s \] is a single point of \(\HH^n_L\), an ordinary vertex of \(P\) when it lies in \(\overline{P}\).

  2. If \(\Sigma_I\) is parabolic of rank \(n-1\), then \(\dim A_I = n\), the form \(\restrictionof{\beta_L}{A_I}\) is negative semidefinite with radical a line \(\RR\eta\) spanned by an isotropic vector \(\eta\), one has \(A_I^{\perp L_\RR} = \RR\eta\), and \[ \Intersect_{s\in I}\overline{H_s} \] meets the closure of \(\HH^n_L\) in the single boundary point \([\eta]\in\partial\HH^n_L\), an ideal vertex of \(P\) when it lies in \(\overline P\). In this case the negative-definite lattice \(\eta^{\perp L}/\eta\) has rank \(n-1\), and the stabilizer of \([\eta]\) in \(W\) is \(W_I\), acting on \((\eta^{\perp L}/\eta)\tensor_\ZZ\RR \cong \EE^{\,n-1}\) as the euclidean reflection group with diagram \(\Sigma_I\).

Proof. Suppose first that \(\Sigma_I\) is elliptic of rank \(n\). Then \(G[I,I]\) is negative definite of rank \(n\), so the \(\alpha_s\) with \(s\in I\) are linearly independent and \(\dim A_I = n\). Since \(\restrictionof{\beta_L}{A_I}\) is nondegenerate, \(L_\RR = A_I\perp A_I^{\perp}\), so \(A_I^{\perp}\) is a line of signature \((1,0)\), spanned by a vector \(v\) with \(v^2 > 0\). Now \(\Intersect_{s\in I}H_s\) is the image of \(A_I^\perp\) in \(\HH^n_L\), a single point.

Suppose instead that \(\Sigma_I\) is parabolic of rank \(n-1\). Each connected component of \(\Sigma_I\) has a one-dimensional kernel by Definition 49.15, so \(\restrictionof{\beta_L}{A_I}\) is negative semidefinite and degenerate. Its radical is \(A_I\intersect A_I^{\perp}\), a totally isotropic subspace of \(L_\RR\), hence of dimension at most \(\min(n_+,n_-) = 1\); being nonzero it is a line \(\RR\eta\) with \(\eta^2 = 0\). Then \(\dim A_I = \rank(\restrictionof{\beta_L}{A_I}) + 1 = n\), so \(\dim A_I^{\perp} = (n+1) - n = 1\), and \(\RR\eta\iscontainedin A_I^\perp\) forces \(A_I^{\perp} = \RR\eta\). The intersection \(\Intersect_{s\in I}\overline{H_s}\) is the image of \(A_I^\perp\) in \(\PP(L_\RR)\), the single point \([\eta]\), which lies on \(\partial\HH^n_L\) because \(\eta\) is isotropic. Since \(\eta\) is isotropic and \(L\) has signature \((1,n)\), the form induced on \(\eta^{\perp}/\eta\) is negative definite of rank \(n-1\); and \(A_I\iscontainedin\eta^{\perp}\), with \(A_I/\RR\eta\) of dimension \(n-1\), so \(W_I\) acts on \(\eta^{\perp}/\eta\) as the reflection group of \(\Sigma_I\), which is euclidean by Definition 50.8.

Corollary 52.7 Sending an ideal vertex \([\eta]\) of \(P\) to the set \(I\) of all walls of \(P\) through \([\eta]\) is a bijection from the ideal vertices of \(P\) onto a set of parabolic subdiagrams of \(\Sigma\) of rank \(n-1\), and each such \(\Sigma_I\) is maximal among the parabolic subdiagrams of \(\Sigma\), in the sense of Definition 49.16.

Proof. Let \([\eta]\) be an ideal vertex and \(I\) the set of walls through it, so \(A_I \iscontainedin \eta^{\perp}\) and \(\Sigma_I\) is parabolic of rank \(n-1\) by Proposition 52.6. Suppose \(\Sigma_J\) is parabolic with \(J\containing I\). Writing \(r\leq 1\) for the dimension of the radical of \(\restrictionof{\beta_L}{A_J}\), one has \(\dim A_J - r = \rank(\restrictionof{\beta_L}{A_J})\leq n_-(L_\RR) = n\), so \(\dim A_J\leq n+1\); and \(\dim A_J = n+1\) would make \(A_J = L_\RR\), whose form is nondegenerate of signature \((1,n)\) rather than negative semidefinite. Hence \(\dim A_J\leq n = \dim A_I\), so \(A_J = A_I\) and every wall indexed by \(J\) contains \(A_I^{\perp} = \RR\eta\), that is, passes through \([\eta]\). By the choice of \(I\) this gives \(J = I\).

Remark 52.8 (The boundary lattice of an ideal vertex) The lattice \(\eta^{\perp L}/\eta\) produced at an ideal vertex by Proposition 52.6 is the quotient appearing in the mirror moves of Definition 51.6, and its rank drops by two from that of \(L\). Reading the same construction one level up, at a \(0\)-cusp \([I]\) of a Baily–Borel compactification, the hyperbolic lattice on which the reflection group acts is \(\overline{T}_I = I^{\perp T}/I\), and the isotropic lines of \(\overline T_I\) are the rank-two isotropic sublattices of \(T\) containing \(I\); Definition 49.16 records these as the \(1\)-cusps adjacent to \([I]\).

52.3 Covolume, compactness, and finite volume

Definition 52.9 Let \(\Gamma\leq\Isom(\HH^n_L)\) be a discrete subgroup. Its covolume is the hyperbolic volume \(\vol(\HH^n_L/\Gamma)\) of the quotient orbifold. If \(\Gamma\) is generated by the reflections in the walls of a Coxeter polytope \(P\), then \(P\) is a strict fundamental domain and \(\vol(\HH^n_L/\Gamma) = \vol(P)\).

Theorem 52.10 (Finite volume and compactness) Let \(P\iscontainedin\HH^n_L\) be a Coxeter polytope with finitely many walls and Coxeter–Vinberg diagram \(\Sigma\).

  1. \(\vol(P) < \infty\) if and only if, in the projective model of Definition 49.9, the closure of \(P\) is the convex hull of finitely many points of \(\overline{\HH^n_L}\) — equivalently, of the ordinary and ideal vertices supplied by Proposition 52.6.

  2. \(P\) is compact if and only if \(\vol(P)<\infty\) and \(P\) has no ideal vertex; equivalently, if and only if the closure of \(P\) in the projective model is the convex hull of finitely many ordinary vertices. By Corollary 52.7 this is the condition that no parabolic subdiagram of \(\Sigma\) of rank \(n-1\) is cut out by a set of walls meeting in \(\overline{P}\).

  3. \(P\) has infinite volume if and only if its closure in the projective model has a vertex outside \(\overline{\HH^n_L}\).

These are Vinberg’s criteria (Vinberg 1967; Vinberg 1985).

Remark 52.11 Criterion (1) is the condition Vinberg’s algorithm tests at each stage (Theorem 49.18, step 4), and Proposition 52.6 converts it into a statement about the subdiagrams of \(\Sigma\): the algorithm terminates when every vertex of the cone bounded by the accepted roots is accounted for by an elliptic subdiagram of rank \(n\) or a parabolic subdiagram of rank \(n-1\).

Corollary 52.12 (The simplex case)

Suppose \(\abs{S} = n+1\) and \(P\) is a simplex, so that its \(n+1\) vertices are cut out by the \(n+1\) maximal proper subdiagrams \(\Sigma_{S\sm\theset{s}}\), \(s\in S\). Then

  1. \(P\) is compact if and only if every \(\Sigma_{S\sm\theset{s}}\) is elliptic;

  2. \(\vol(P) < \infty\) if and only if every \(\Sigma_{S\sm\theset{s}}\) is elliptic or parabolic, the ideal vertices being those \(s\) for which \(\Sigma_{S\sm\theset{s}}\) is parabolic;

  3. \(\vol(P) = \infty\) if and only if some \(\Sigma_{S\sm\theset{s}}\) is hyperbolic.

Remark 52.13 (Lannér's classification) The compact hyperbolic Coxeter simplices of Corollary 52.12(1) were classified by Lannér. In \(\HH^2\) they are the triangles of Example 52.14 with \(p,q,r\) all finite, of which there are infinitely many; in \(\HH^3\) there are nine and in \(\HH^4\) there are five.

Example 52.14 (Triangle groups) Let \(2\leq p\leq q\leq r\leq\infty\) and let \(\Sigma\) be the rank-three diagram with \(m_{12} = p\), \(m_{13} = q\), \(m_{23} = r\). A Coxeter polytope realizing \(\Sigma\) is a triangle with angles \(\pi/p\), \(\pi/q\), \(\pi/r\), an angle \(0\) being read as an ideal vertex, and the type of \(\Sigma\) is decided by the angle sum: \[ \frac1p + \frac1q + \frac1r \;\begin{cases} > 1 & \text{spherical, and the triangle lies in } S^2;\\ = 1 & \text{euclidean, and the triangle lies in } \EE^2;\\ < 1 & \text{hyperbolic, and the triangle lies in } \HH^2. \end{cases} \] In the hyperbolic case the triangle has area \(\pi\left(1 - \tfrac1p - \tfrac1q - \tfrac1r\right)\) by the Gauss–Bonnet theorem, so every hyperbolic triangle group has finite covolume; it is cocompact exactly when \(p,q,r\) are all finite, and each entry equal to \(\infty\) contributes one ideal vertex. The spherical solutions are \((2,2,r)\), \((2,3,3)\), \((2,3,4)\) and \((2,3,5)\), which are the rank-three entries \(I_2(r)\times A_1\), \(A_3\), \(B_3\) and \(H_3\) of Theorem 50.17; the euclidean solutions are \((2,3,6)\), \((2,4,4)\) and \((3,3,3)\), which are \(\tilde G_2\), \(\tilde C_2\) and \(\tilde A_2\).

Remark 52.15 (Dihedral angles and the angle sum) Every dihedral angle of a Coxeter polytope is \(\pi/m\) with \(m\geq 2\) by Definition 49.11, hence at most \(\pi/2\), in each of the three geometries: a Coxeter polytope is acute-angled. What separates the three cases in Example 52.14 is the angle sum compared with the euclidean value \(\pi\), and in higher rank the definiteness of the Gram form (Definition 50.8).

52.4 The cusps of the quotient

Remark 52.16 (Ends of the quotient orbifold) Let \(\Gamma\) be generated by the reflections in the walls of a finite-volume Coxeter polytope \(P\). Since \(P\) is a strict fundamental domain, the quotient orbifold \(\HH^n_L/\Gamma\) is homeomorphic to \(P\), and its ends — its cusps — are the ideal vertices of \(P\). By Proposition 52.6 the cross-section of the cusp at \([\eta]\) is the euclidean orbifold \(\EE^{\,n-1}/W_I\), where \(\Sigma_I\) is the parabolic subdiagram of rank \(n-1\) cutting out \([\eta]\); it is compact because \(W_I\) is a euclidean reflection group of full rank, and its volume is finite, which is why an ideal vertex contributes finitely to \(\vol(P)\). A vertex lying outside \(\overline{\HH^n_L}\) contributes an end of infinite volume.

Remark 52.17 (Invariants computed from the diagram) The data separating these cases — the dimension, the number of vertices at infinity, the \(f\)-vector of the polytope, the covolume through the Euler characteristic in even dimension, and the growth series of the Coxeter system — are computed from the Coxeter–Vinberg diagram alone by Guglielmetti’s implementation of Vinberg’s criteria (Guglielmetti 2015). The runs recorded for the lattices of this monograph are collected in the Computations section.