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49  Hyperbolic reflection groups and Vinberg’s algorithm

Remark 49.1 (Orientation and sign convention) The Baily–Borel and semitoroidal compactifications of the period domains attached to Coble and Enriques surfaces are governed by the action of arithmetic reflection groups on hyperbolic space. This section collects the reflection-group and hyperbolic-polytope combinatorics used elsewhere in the monograph, culminating in Vinberg’s algorithm for a fundamental chamber and the Coxeter–Vinberg diagram that encodes it.

We work throughout with the algebraic-geometry sign convention (see the Lattice Theory section): roots are negative-definite directions, a short root having norm \(v^2 = -2\) and a long root norm \(v^2 = -4\). An indefinite lattice \(L\) of signature \((1, n)\) is called hyperbolic; its positive cone \[ C_L \definedas \theset{ v\in L_\RR \mid v^2 > 0 } \] has two connected components, and mirrors of roots are the hyperplanes orthogonal to negative-norm vectors. Fixing one component \(C_L^+\) identifies \(L_\RR\) with a model of hyperbolic \(n\)-space, on which the reflection group acts.

49.1 Reflections and the Weyl group

Definition 49.2 Let \((L,\beta_L)\) be a nondegenerate integral lattice and \(\alpha\in L\) an anisotropic vector (that is, \(\beta_L(\alpha,\alpha)\neq 0\)) satisfying the integrality condition \[ 2\,\beta_L(\alpha, L) \iscontainedin \beta_L(\alpha,\alpha)\,\ZZ . \] The reflection in \(\alpha\) is the isometry \[ \begin{aligned} s_\alpha : L &\to L \\ x &\mapsto x - 2\,\frac{\beta_L(\alpha,x)}{\beta_L(\alpha,\alpha)}\,\alpha . \end{aligned} \] Each reflection has determinant \(-1\), fixes the orthogonal complement \(\generators{\alpha}^{\perp L}\) pointwise, and sends \(\alpha\mapsto -\alpha\). The integrality condition is exactly what guarantees \(s_\alpha\in\Orth(L)\) rather than merely \(s_\alpha\in\Orth(L_\QQ)\). The Weyl group of \(L\) is the subgroup \[ W(L) \definedas \generators{\, s_\alpha \mid \alpha\in L \text{ anisotropic},\ s_\alpha\in\Orth(L) \,} \leq \Orth(L) \] generated by all such integral reflections.

49.2 Roots and \(k\)-roots

Definition 49.3 Let \(L\) be a nondegenerate integral lattice and \(v\in L\) a primitive vector. The reflection \(s_v\) is always an isometry of \(L_\QQ\), but is integral, \(s_v\in\Orth(L)\), precisely when \[ 2\,\beta_L(v, L) \iscontainedin v^2\,\ZZ . \] For \(k>0\) the \(k\)-Weyl group \(W^k(L)\) is the subgroup generated by the integral reflections \(s_v\) with \(v^2 = -k\): \[ W^k(L) \definedas \generators{\, s_v \mid s_v\in\Orth(L),\ v^2 = -k \,} . \] A vector \(v\) is a \(k\)-root if \(s_v\in W^k(L)\). The full Weyl group \(W(L)\) of Definition 49.2 is generated by integral reflections in vectors of any norm, so that \(W(L) = \generators{\, W^k(L) \mid k>0 \,}\).

Definition 49.4

For a \(2\)-elementary lattice \(L\), the roots are precisely the vectors \(v\in L\) of one of the two following types:

  • a short root, with \(v^2 = -2\); or

  • a long root, with \(v^2 = -4\) and \(\div_L(v) = 2\).

Remark 49.5 The two root types of Definition 49.4 are exactly the primitive vectors of norm \(-2\) or \(-4\) satisfying the integrality condition of Definition 49.3. For a short root, \(2\beta_L(v,L)\iscontainedin 2\ZZ = v^2\ZZ\) holds automatically, and \(s_v(x) = x + \beta_L(v,x)\,v\). For a norm \(-4\) vector, integrality requires \(\beta_L(v,L)\iscontainedin 2\ZZ\), which is precisely the condition \(\div_L(v) = 2\); then \(s_v(x) = x + \tfrac12\beta_L(v,x)\,v\) lies in \(\Orth(L)\).

Theorem 49.6 (The two root types exhaust the roots) Let \(L\) be an even \(2\)-elementary lattice and let \(v\in L\) satisfy \(v^2 < 0\). Then \(s_v\in\Orth(L)\) if and only if either \(v^2 = -2\), or \(v^2 = -4\) and \(\div_L(v) = 2\). The list of Definition 49.4 is therefore exhaustive, not merely a selection of two convenient cases.

Proof. This is (Alexeev 2022, sec. 2); that the two conditions are sufficient is Remark 49.5, and the content of the theorem is that no other norm occurs. For \(L\) \(2\)-elementary one has \(\div_L(v)\in\theset{1, 2}\), so the integrality condition \(2\div_L(v)/v^2\in\ZZ\) of Definition 49.3 restricts \(v^2\) to \(\theset{-2, -4}\), with \(\div_L(v) = 2\) forced in the second case.

49.3 Coroots and the weight space

Definition 49.7 Let \(R \definedas \generators{\Phi}_\ZZ\) be a root lattice generated by a root system \(\Phi = \theset{\alpha_1,\dots,\alpha_n}\). The dual lattice \(\dualof{R}\) is called the weight lattice of \(R\), and the dual-basis elements \(\omega_i\) determined by \((\omega_i, \alpha_j) = \delta_{ij}\) are the fundamental weights.

Remark 49.8 For the unimodular root lattice \(E_8\) one has \(\dualof{E_8}\cong E_8\), and the fundamental weights \(\omega_1,\dots,\omega_8\) — the dual basis to the simple roots — have coordinates in the simple-root basis given by the rows of the inverse Gram matrix. The coroots are \(\dualof{\alpha_i} = 2\alpha_i/(\alpha_i,\alpha_i)\), which for the simply-laced \(E_8\) coincide with the simple roots themselves — distinct from the \(\omega_i\).

49.4 Hyperbolic lattices and their polytopes

Definition 49.9

Let \(L\) be a hyperbolic lattice and \(C_L^+\) one connected component of its positive cone \(C_L\). The intersection pairing induces a hyperbolic metric on \(L_\RR\), and the associated hyperbolic \(n\)-space \(\HH^n_L\) admits two standard models:

  • the unit hyperboloid model \[ \HH^n_L \definedas \theset{ v\in L_\RR \mid v^2 = 1,\ v\in C_L^+ } ; \]

  • the projective (Klein) model \[ \BB^n_L \definedas \PP(C_L) \subset \PP(L_\RR) , \] the image of the positive cone in projective space.

In the projective model the boundary \(\partial\HH^n_L\) is the image \(\PP(\theset{v\in L_\RR \mid v^2 = 0})\) of the isotropic cone, geodesics are straight line segments, and angles are distorted. The Poincaré ball and upper half-space charts are the two standard conformal models, in which dihedral angles agree with the angles between the bounding hyperplanes and geodesics are circular arcs meeting the boundary sphere orthogonally.

The hyperbolic distance between points is recovered from the form: for \(v, w\in C_L^+\), \[ \cosh d\!\left([v],[w]\right) = \frac{\beta_L(v,w)}{\sqrt{v^2\,w^2}} , \] the right-hand side being at least \(1\) by the reverse Cauchy–Schwarz inequality on the positive cone, with equality exactly when \([v] = [w]\). On the unit hyperboloid, where \(v^2 = w^2 = 1\), this reads \(\cosh d(v,w) = \beta_L(v,w)\).

To a vector \(v\) with \(v^2 < 0\) we associate the hyperplane \(H_v \definedas v^{\perp}\intersect \HH^n_L\). Taking \(v,w\) to be unit normals of \(H_v, H_w\), the relative position of two hyperplanes is governed by the pairing \(v\cdot w \definedas \beta_L(v,w)\):

  • \(|v\cdot w| < 1\): the hyperplanes meet in the interior of \(\HH^n_L\), at dihedral angle \(\theta\) with \(-v\cdot w = \cos\theta\);

  • \(|v\cdot w| = 1\): the hyperplanes are asymptotically parallel, meeting at an ideal point of the boundary \(\partial\HH^n_L\);

  • \(|v\cdot w| > 1\): the hyperplanes are ultra-parallel (divergent), at distance \(d\) with \(-v\cdot w = \cosh d\).

49.5 Coxeter groups and Coxeter polytopes

Definition 49.10 A Coxeter group is a group given by a presentation \[ W = \generators{\, s_1,\dots,s_n \mid s_i^2 = e,\ (s_i s_j)^{m_{ij}} = e \,} , \] where \(m_{ii} = 1\) and \(m_{ij} = m_{ji}\in\theset{2,3,\dots}\union\theset{\infty}\) for \(i\neq j\), the relation \((s_i s_j)^{m_{ij}} = e\) being imposed only when \(m_{ij} < \infty\). Writing \(S = \theset{s_1,\dots,s_n}\) for the generating set, the pair \((W,S)\) is the associated Coxeter system, and \((m_{ij})\) is its Coxeter matrix (Definition 50.2).

Definition 49.11

A Coxeter polytope \(P\) in \(X = \EE^n\), \(S^n\), or \(\HH^n\) is a polytope such that:

  1. each face lies in a hyperplane \(H_i \definedas \alpha_i^{\perp}\);

  2. the dihedral angles between adjacent faces are \(\pi/m_{ij}\) for integers \(m_{ij}\geq 2\); and

  3. the reflection group \(W = \generators{ s_{\alpha_i} }\) acts properly discontinuously on \(X\) with fundamental domain \(P\).

49.6 The discriminant locus and Weyl chambers

Definition 49.12 The discriminant locus \(\Delta(L)\) is the union of all mirrors, i.e. the hyperplanes orthogonal to the short roots \(\Phi_2(L) = \theset{ v\in L \mid v^2 = -2 }\): \[ \Delta(L) \definedas \Union_{v\in\Phi_2(L)} v^{\perp L} . \]

Definition 49.13 The Weyl chambers of \(L\) are the connected components of \[ C_L^{\circ} \definedas C_L \sm \Delta(L) , \] where \(C_L\) is the cone of positive-norm vectors. A simple system is a set of roots whose mirrors form the bounding hyperplanes of a single Weyl chamber, i.e. a fundamental domain for the action of the \(2\)-Weyl group \(W^2(L)\) generated by the \((-2)\)-reflections.

49.7 Coxeter–Vinberg diagrams

Definition 49.14 The Coxeter–Vinberg diagram of a hyperbolic Coxeter polytope is a graph whose vertices correspond to the simple roots \(r_i\) (equivalently, to the walls \(H_i\) of the fundamental chamber), and whose edges encode the relative position of the mirrors \(H_i, H_j\) through the normalized pairing \[ g_{ij} \definedas \frac{r_i\cdot r_j}{\sqrt{r_i^2\, r_j^2}} = \cos\!\left(\frac{\pi}{m_{ij}}\right) . \] Edges are drawn according to the following conventions:

  • omitted if \(m_{ij}\leq 2\) (orthogonal mirrors);

  • multiple (or labeled) lines if \(3\leq m_{ij} < \infty\), when \(H_i\intersect H_j\) lies in the interior of \(\HH^n\);

  • thick / bold if \(g_{ij} = 1\), when \(H_i\intersect H_j\) lies on the boundary \(\partial\HH^n\) (asymptotically parallel mirrors);

  • dotted if \(g_{ij} > 1\) (ultra-parallel mirrors).

For \(2\)-elementary lattices, vertices are drawn white for short roots (\(v^2 = -2\)) and black for long roots (\(v^2 = -4\)).

49.8 Elliptic and parabolic subdiagrams

Definition 49.15 Let \(G\) be a Coxeter–Vinberg diagram with vertices the simple roots \(v_i\). A subdiagram \(D\iscontainedin G\) is:

  • elliptic if its Gram matrix \((v_i\cdot v_j)\) is negative definite; such a subdiagram defines a finite face of the Coxeter polytope;

  • parabolic if the Gram matrix of each of its connected components is negative semidefinite with a one-dimensional kernel;

  • hyperbolic if its Gram matrix is nondegenerate of signature \((1, k-1)\), where \(k\) is the number of vertices of \(D\).

The signs follow the algebraic-geometry convention of this monograph, in which roots have negative norm; in the normalization by roots of norm \(+2\) the three conditions read positive definite, positive semidefinite of corank one, and signature \((k-1,1)\).

Definition 49.16 A parabolic subdiagram is maximal if it is not properly contained in a larger parabolic subdiagram.

Let \(L\) have signature \((2,n)\), let \(I\subset L\) be a primitive isotropic line, and let \(\overline{L}_I \definedas I^{\perp L}/I\) be the hyperbolic lattice at the \(0\)-cusp \([I]\). The maximal parabolic subdiagrams of the Coxeter–Vinberg diagram of a fundamental chamber for a reflection group acting on \(\overline{L}_I\) correspond bijectively to the primitive isotropic lines of \(\overline{L}_I\) met by that chamber, that is, to the rank-\(2\) primitive isotropic sublattices \(J\subseteq L\) containing \(I\); passing to orbits under the stabilizer of \(I\) in the arithmetic group gives the \(1\)-cusps adjacent to the \(0\)-cusp \([I]\) in the Baily–Borel compactification.

Remark 49.17 The bijection is the vertex correspondence of Proposition 52.6: a maximal parabolic subdiagram of rank \(n-1\) is cut out by the walls through an ideal vertex of the chamber, and that ideal vertex is the isotropic line spanned by the radical of the subdiagram.

49.9 Vinberg’s algorithm

Theorem 49.18 (Vinberg’s algorithm for a fundamental chamber) Let \(\Gamma\iscontainedin\Orth(L)\) be an arithmetic reflection subgroup of a hyperbolic lattice \(L\). Vinberg’s algorithm constructs a fundamental chamber \(P\iscontainedin\HH^n\) for \(\Gamma\) as follows (Vinberg 1975).

  1. Control vector. Choose a generic positive-norm vector \(v_0\in C_L^+\). Its stabilizer in \(\Gamma\) is the reflection group of the finite subdiagram of roots orthogonal to \(v_0\), which are used as the first walls.

  2. Enumeration by height. Seek primitive vectors \(v\in L\) with \(v^2 < 0\), \(s_v\in\Gamma\), and mirror \(H_v\) meeting the current cone \(\tilde P\). Order the candidate roots by increasing height \[ h(v) \definedas \frac{-2\,(v_0\cdot v)}{v^2} , \] a quantity monotone in the hyperbolic distance from \(v_0\) to \(H_v\) (for roots of a given norm), used to order the candidate roots and process them in successive batches of increasing height.

  3. Acceptance / pruning. A candidate root \(v\) at the current height is kept only if its mirror bounds the chamber on the side containing \(v_0\); equivalently, the inward normals of accepted walls pair non-negatively, so \(v\) is accepted iff \(v\cdot r_i \geq 0\) for every previously accepted root \(r_i\) (a mirror with some \(v\cdot r_i < 0\) would cut into the interior of \(\tilde P\)). Roots whose hyperplanes do not bound \(\tilde P\) are discarded.

  4. Diagram. The algorithm proceeds until \(\tilde P\) becomes a convex polytope of finite volume. The Gram matrix of the accepted simple roots then determines the Coxeter–Vinberg diagram of \(P\) (Definition 49.14).

The algorithm terminates in finitely many steps if and only if \(\Gamma\) has finite covolume in \(\Orth(L)\).

Remark 49.19 The source note states step 3 as “discard those roots whose hyperplanes do not intersect \(\tilde P\) transversely”; the explicit inequality \(v\cdot r_i\geq 0\) recorded above is Vinberg’s acceptance criterion for that condition in the algebraic-geometry sign convention (Vinberg 1975).

Definition 49.20 (Lannér subgraph) A Lannér subgraph of a Coxeter–Vinberg diagram is a minimal non-parabolic subdiagram whose Coxeter group acts on hyperbolic space with a fundamental polyhedron of finite volume; equivalently, a diagram all of whose proper subdiagrams are elliptic but which is not itself elliptic or parabolic.

Theorem 49.21 (A sufficient criterion for completeness) Let \(\Gamma\) be a Coxeter–Vinberg diagram without dotted edges, obtained from a run of Theorem 49.18. If \(\Gamma\) contains no Lannér subgraph, and every connected parabolic subdiagram of \(\Gamma\) is contained in a maximal parabolic subdiagram of maximal rank, then \(\Gamma\) is complete: the accepted roots bound a finite-volume polytope and the algorithm may be stopped.

Proof. This is the sufficient condition of (Vinberg 1975), in the form used in (Alexeev 2022, sec. 3).

Remark 49.22 The criterion is what makes a run of Vinberg’s algorithm into a proof rather than a computation that has not yet terminated: without it, the algorithm has produced a chamber that might still acquire further walls at greater height. It applies only in the absence of dotted edges, that is when no two walls of the chamber are ultra-parallel; for the diagrams on the line \(r + a = 22\) this is the case, and Theorem 53.11 records the resulting diagrams.

49.10 Root-system conventions and tables

Remark 49.23 (Bourbaki conventions and twists) The classical root systems are taken with the Bourbaki labeling, in the negative-definite twists appropriate to the algebraic-geometry convention: \[ A_n(-1),\quad B_n(-2),\quad C_n(-1),\quad D_n(-1),\quad E_n(-1)\ (n\in\theset{6,7,8}),\quad F_4(-2),\quad G_2(-1) . \] Both the simply-laced systems \(A_n, D_n, E_n\) and the non-simply-laced systems \(B_n, C_n, F_4, G_2\) occur. The Coxeter diagrams and Gram matrices used throughout are derived from these conventions; the ranks, root counts, Weyl groups, discriminant groups, and explicit Euclidean basis constructions are collected in the root-lattice tables of the Root Systems section and the Diagrams appendix.