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53  Reflective two-elementary lattices

Remark 53.1 (Orientation) Every lattice in [[coble-lattice-table|the Coble table]] satisfies \(r + a = 22\): the row indexed by \(n\) has \(r = 10 + n\) and \(a = 12 - n\). The line \(r + a = 22\) is exactly the range in which Alexeev classifies the reflective even hyperbolic \(2\)-elementary lattices and computes their Coxeter diagrams and maximal parabolic subdiagrams (Alexeev 2022), so that classification is a classification of the Coble Picard lattices. This section records it, together with the reflection-theoretic vocabulary it needs, and the consequences for the automorphism groups of the K3 covers.

We work with the sign convention of the Hyperbolic Reflection Groups section: hyperbolic means signature \((1, r-1)\), root lattices are negative definite, and the roots of an even \(2\)-elementary lattice are the short roots \(v^2 = -2\) and the long roots \(v^2 = -4\) with \(\div_L(v) = 2\) (Definition 49.4).

53.1 Reflective and two-reflective lattices

Definition 53.2 (Reflective and two-reflective) Let \(S\) be an even hyperbolic lattice, and let \(W_2(S)\leq W_r(S)\leq\Orth(S)\) be the subgroups generated by the reflections in the short roots, respectively in all roots, in the notation of Definition 49.3. Both are normal in \(\Orth(S)\) and in its index-two subgroup \(\Orth^+(S)\) preserving the positive cone. The lattice \(S\) is two-reflective if \(W_2(S)\) has finite index in \(\Orth(S)\), and reflective if \(W_r(S)\) has finite index in \(\Orth(S)\). Write \(\Gamma_2\) and \(\Gamma_r\) for the Coxeter–Vinberg diagrams (Definition 49.14) of the two reflection groups, and \(\Gamma_4\iscontainedin\Gamma_r\) for the subdiagram spanned by the long roots.

Remark 53.3 Vinberg’s algorithm (Theorem 49.18) terminates on \(S\) for the group \(W_r(S)\) exactly when \(S\) is reflective, and for \(W_2(S)\) exactly when \(S\) is two-reflective; \(\Gamma_r\) is in practice far smaller than \(\Gamma_2\), so it is the diagram to compute (Alexeev 2022, sec. 1). Reflectivity is a strong restriction: \(a\leq r\) always, and a reflective lattice satisfies \(r\leq 20\) or \(r = 22\), so there are only finitely many reflective \(2\)-elementary lattices (Alexeev 2022, sec. 1).

Definition 53.4 (Coeven and coodd lattices) For a \(2\)-elementary lattice \(S\), the doubled dual is \(S^{\dagger}\definedas \dualof{S}(2)\), the dual lattice of Theorem 37.13 with its form scaled by \(2\). The lattice \(S\) is coeven if \(S^{\dagger}\) is even and coodd if \(S^{\dagger}\) is odd; these are the cases \(\delta = 0\) and \(\delta = 1\) of the coparity invariant of the Lattice Theory section. A direct sum of \(2\)-elementary lattices is coeven if and only if every summand is.

Theorem 53.5 (Duality for coeven lattices) If \(S\) is even hyperbolic \(2\)-elementary and coeven with invariants \((r, a, 0)\), then \(S^{\dagger} = \dualof{S}(2)\) is again even hyperbolic and coeven, with invariants \((r, r - a, 0)\). The Coxeter diagrams \(\Gamma_r(S)\) and \(\Gamma_r(S^{\dagger})\) are dual, in that the short and long roots are interchanged, and this duality induces a bijection between the maximal parabolic subdiagrams of the two, interchanging the odd and even ordinary types of Theorem 35.14.

Proof. This is (Alexeev 2022, sec. 3).

53.2 The decomposition of even hyperbolic two-elementary lattices

Theorem 53.6 (Elementary summands) Every even hyperbolic \(2\)-elementary lattice is an orthogonal direct sum of exactly one of the elementary hyperbolic summands \[ \generators{2} = (1,1,1), \qquad U = (2,0,0), \qquad U(2) = (2,2,0) , \] together with a direct sum of negative definite root lattices drawn from the list \[ B_n(2) = (n, n, 1), \quad C_{4n} = (4n, 2, 0)\ (4n\geq 8), \quad C_{4n+2} = (4n+2, 2, 1)\ (4n+2\geq 6), \] \[ F_4 = (4, 2, 0), \qquad E_7 = (7,1,1), \qquad E_8 = (8,0,0), \qquad E_8(2) = (8,8,0) , \] each listed with its invariants \((r, a, \delta)\). Here the non-simply-laced names refer to the full reflection group of the summand; as \((-2)\)-root lattices in the sense of Definition 36.2 one has \[ B_n(2) = A_1^{\oplus n}, \qquad C_n = D_n, \qquad F_4 = D_4 . \]

Proof. This is (Alexeev 2022, sec. 2).

Remark 53.7 Theorem 53.6 is the exhaustive form of the building-block statement of Theorem 39.21: in the \((-2)\)-root translation, the negative definite summands available are exactly \(A_1^{\oplus n}\), \(D_{2m}\) for \(2m\geq 4\), \(E_7\), \(E_8\) and \(E_8(2)\). The lattices \(D_{2m+1}\) and \(E_6\) do not occur, their discriminant groups being \(\ZZ/4\ZZ\) and \(\ZZ/3\ZZ\), which are not \(2\)-elementary. Each entry of the Coble table is of this shape: for instance the \(n = 1\) row is \(S_\Co = \generators{-2}\oplus E_{10}(2) = U(2)\oplus\bigl(A_1\oplus E_8(2)\bigr)\), with hyperbolic summand \(U(2)\) and definite part \(A_1\oplus E_8(2)\).

53.3 Main roots and diagrams built on a graph

Definition 53.8 (Main roots) Let \(L\) be an even \(2\)-elementary lattice. The main roots of \(L\) are the short roots \(\alpha\), \(\alpha^2 = -2\), of divisibility \(\div_L(\alpha) = 1\), that is those satisfying \(\beta_L(\alpha, L) = \ZZ\).

Definition 53.9 (The edge $n$-fold graph) Let \(G\) be a graph and \(n\geq 2\) an integer. The edge \(n\)-fold graph \(G^{(n)}\) is obtained from \(G\) by subdividing each edge into \(n\) edges, inserting \(n-1\) new vertices on each. Thus \(G^{(n)}\) has \(n\abs{E_G}\) edges and \(\abs{V_G} + (n-1)\abs{E_G}\) vertices, where \(E_G\) and \(V_G\) are the edge and vertex sets of \(G\). The vertices of \(G^{(2)}\) are in natural bijection with the vertices and edges of \(G\) together.

Definition 53.10 (Built on top of a graph) A Coxeter–Vinberg diagram \(\Gamma\) is built on top of a simple graph \(H\) if \(\Gamma\) contains a subdiagram of main roots isomorphic to \(H\) and the remaining roots of \(\Gamma\) are determined by the main roots by explicit rules. For the lattices of Theorem 53.11 the rule is that a main root \(\alpha\) and an additional root \(\beta\) are joined exactly when \(\alpha\cdot\beta = 2\).

Theorem 53.11 (Coxeter diagrams on the line $r + a = 22$) For \(n = 3, 4, 5, 6\) the Coxeter diagram \(\Gamma_r\) of the even \(2\)-elementary lattice \((10 + n,\, 12 - n,\, 1)\) is built on top of \(K_n^{(2)}\), where \(K_n\) is the complete graph on \(n\) vertices. The diagram of \((18, 4, 0)\) is built on top of \(K_{4,4}^{(2)}\), the complete bipartite graph on \(4 + 4\) vertices, and that of \((14, 8, 0)\) on top of \(D_4^{(2)}\). In every case \[ \Aut\Gamma_r = \Aut G, \qquad \Orth^+(S) = \Aut G \ltimes W_r(S) , \] where \(G\) is the graph in question.

Proof. This is (Alexeev 2022 Thm. 1.4).

Remark 53.12 The lattices \((10+n, 12-n, 1)\) of Theorem 53.11 are precisely the rows \(n\) of the Coble table, so for \(n = 3, 4, 5, 6\) the Coxeter diagram of the Picard lattice of the K3 cover is built on the doubled complete graph \(K_n^{(2)}\), and the full isometry group of the Picard lattice is the reflection group extended by the symmetric group \(\Aut K_n = S_n\) permuting the \(n\) boundary components.

Theorem 53.13 (Non-reflective lattices on the line $r + a = 22$) The even \(2\)-elementary lattices \((17, 5, 1)\), \((18, 4, 1)\) and \((19, 3, 1)\) are not reflective. Every other even \(2\)-elementary lattice on the line \(r + a = 22\) is reflective.

Proof. This is (Alexeev 2022 Thm. 1.1).

Remark 53.14 The three non-reflective lattices are the Picard lattices of the rows \(n = 7, 8, 9\) of the Coble table: \((17,5,1)\) at \(n = 7\), \((18,4,1)\) at \(n = 8\), and \((19,3,1)\) at \(n = 9\). For these three families the reflection group \(W_r\) has infinite index in \(\Orth(S)\), Vinberg’s algorithm does not terminate, and there is no finite Coxeter diagram to fold; the toroidal compactifications built from Coxeter fans are unavailable in exactly these three cases. The remaining eight rows, including the case \(n = 1\) of principal interest, are reflective.

53.4 Maximal parabolic subdiagrams and cusps

Theorem 53.15 (Maximal parabolic subdiagrams classify isotropic vectors) Let \(S\) be a reflective even hyperbolic lattice, so that Vinberg’s algorithm terminates and produces a finite diagram \(\Gamma_r\). Then the \(\Orth(S)\)-orbits of primitive isotropic vectors of \(S\) correspond bijectively to the maximal parabolic subdiagrams of \(\Gamma_r\), and the orbits modulo \(\Aut\Gamma_r\) to the subdiagrams modulo \(\Aut\Gamma_r\). If a parabolic subdiagram \(\Gamma\iscontainedin\Gamma_r\) corresponds to \(v\in S\) with \(v^2 = 0\), then the image of \(\Gamma\) in \(v^{\perp}/v\) spans an elliptic root system: of type \(A_n, D_n, E_n\) when every vertex of \(\Gamma\) is a short root, and of type \(A_n(2), D_n(2), E_n(2)\) when every vertex is a long root.

Proof. This is (Alexeev 2022, sec. 1, §2).

Remark 53.16 Under the two interpretations of an even hyperbolic lattice, this correspondence reads twice. When \(S = S_X\) is the Picard lattice of a K3 surface, its primitive isotropic vectors are the elliptic fibrations of \(X\) (Theorem 53.24), so the maximal parabolic subdiagrams of \(\Gamma_r(S_X)\) classify the elliptic fibrations up to \(\Orth(S_X)\). When \(S = \delta^{\perp}/\delta\) is the hyperbolic lattice attached to a \(0\)-cusp of a type IV quotient, its isotropic vectors are the \(1\)-cusps adjacent to that \(0\)-cusp, and the corresponding Type II degenerations (Alexeev 2022, sec. 1); this is the reading used in Definition 49.16.

Theorem 53.17 (Counts of maximal parabolic subdiagrams)

For the reflective even \(2\)-elementary lattices with \(r + a\geq 22\) the numbers of maximal parabolic subdiagrams of \(\Gamma_r\), in total and modulo \(\Aut\Gamma_r\), are the following.

Table 53.1: Maximal parabolic subdiagram counts
Lattice Total Modulo \(\Aut\Gamma_r\)
\((11,11,1)\) 2 2
\((12,10,1)\) 5 5
\((13,9,1)\) 22 7
\((14,8,0)\) 11 5
\((14,8,1)\) 127 15
\((15,7,1)\) 1027 20
\((16,6,1)\) 8917 28
\((18,4,0)\) 5244 17
\((20,2,1)\) 581 13
\((13,11,1)\) 9 5
\((14,10,1)\) 69 12
\((15,9,1)\) 2114 20
\((18,6,0)\) 90,897,634 28
\((22,2,0)\) 1,095,990 18
\((14,12,1)\) 16 9
\((15,11,1)\) 522 16
\((15,13,1)\) 46 10
\((16,14,1)\) 115 14

Proof. This is (Alexeev 2022, sec. 3).

Theorem 53.18 (The Coxeter diagram of $S_\Co$) The Coxeter diagram \(\Gamma_r\) of \(S_\Co = (11,11,1)_1\) has \(12\) roots and trivial automorphism group \(\Aut\Gamma_r = 1\). It has exactly two maximal parabolic subdiagrams, \[ \wE_8(2)\,\wA_1 \qquad\text{and}\qquad \wB_9(2) , \] both of even ordinary type, so there are two \(\Orth^+(S_\Co)\)-orbits of primitive isotropic vectors in \(S_\Co\).

Proof. This is (Alexeev 2022, sec. 3), the case \((11,11,1)\) of Theorem 53.17.

Remark 53.19 (What the two orbits do and do not count) Both subdiagrams of Theorem 53.18 have rank \(9 = r - 2\), as a maximal parabolic subdiagram of a rank-\(11\) hyperbolic lattice must, and both are of even ordinary type, so by Theorem 35.14 each associated quotient \(v^{\perp}/v\) has invariants \((9, 9, 1)\). The two orbits are therefore not separated by the invariants of the quotient lattice.

These are orbits of isotropic vectors in the algebraic lattice \(S_\Co\), which by Theorem 53.24 count the elliptic fibrations of the K3 cover of a Coble surface. They are not the \(0\)-cusps of the Coble period space: those are the orbits of primitive isotropic vectors in the transcendental lattice \(T_\Co\) of signature \((2,9)\), of which there is exactly one under the full orthogonal group (Theorem 85.27). The two lattices share the invariants \((11,11,1)\) but not the signature, and the counts attached to them are different counts.

53.5 Reflection groups and automorphisms of the K3 cover

Theorem 53.20 (The quotient $W_r/W_2$) For an even hyperbolic \(2\)-elementary lattice \(S\) the quotient group \(W_r(S)/W_2(S)\) is isomorphic to the Coxeter group \(W(\Gamma_4)\) generated by the reflections in the long roots.

Proof. This is (Alexeev 2022, sec. 4).

Theorem 53.21 (The automorphism group of a two-elementary K3 surface) Let \(S\neq\EnriquesInvariants\) be an even \(2\)-elementary hyperbolic lattice admitting a primitive embedding into \(\lkt\), let \(X\) be a K3 surface with \(S_X = S\), let \(\iota\) be the canonical nonsymplectic involution of \(X\), let \(Y = X/\iota\) with branch divisor \(B\), and let \(\Aut'(Y, B)\) be the subgroup of automorphisms of the pair \((Y, B)\) acting trivially on \(\Pic Y\). Then there is an exact sequence \[ 0 \to \generators{\iota}\times\Aut'(Y, B) \to \Aut(X) \to \Sym\Gamma_r \ltimes W(\Gamma_4) \to 0 , \] where \(\Gamma_4\iscontainedin\Gamma_r\) is the subdiagram of long roots. In particular \(\Aut(X)\) is finite if and only if \(S_X\) is two-reflective.

Proof. This is (Alexeev 2022 Lem. 4.1). The proof combines three inputs: the Torelli theorem, by which \(\Aut(X)\to\Orth^+(S)/W_2(S)\) has finite kernel and cokernel and the kernel consists of the automorphisms descending to \((Y, B)\); the surjectivity of \(\Orth(H)\to\Orth(A_H, q_H)\) for indefinite \(2\)-elementary \(H\) (Theorem 37.23), which lifts an isometry of \(S\) to one of \(\lkt\); and Theorem 53.20, which identifies \(\Orth^+(S)/W_2(S)\) with \(\Sym\Gamma_r\ltimes W(\Gamma_4)\).

Theorem 53.22 (Long-root subdiagrams on the line $r + a = 22$)

For the reflective even \(2\)-elementary lattices on the line \(r + a = 22\), the diagram automorphism group and the long-root subdiagram \(\Gamma_4\) of Theorem 53.21 are the following, where \(T_{p,q,r}\) denotes a tree with three legs of lengths \(p\), \(q\), \(r\).

Table 53.2: Long-root subdiagrams \(\Gamma_4\)
Lattice \(\Aut\Gamma_r\) \(\Gamma_4\)
\((11,11,1)\) \(1\) \(T_{2,3,7}\)
\((12,10,1)\) \(1\) \(T_{2,4,6}\sqcup A_1\)
\((13,9,1)\) \(S_3\) \(T_{4,4,4}\)
\((14,8,1)\) \(S_4\) trivalent tree on \(10\) vertices with \(6\) ends
\((15,7,1)\) \(S_5\) Petersen graph
\((16,6,1)\) \(S_6\) diagram on \(10 + 15\) vertices
\((18,4,0)\) \(S_2\ltimes(S_4\times S_4)\) diagram on \(24\) vertices
\((20,2,1)\) \(S_5\) \(K_5\) with bold edges

Proof. This is (Alexeev 2022 Table 1).

Remark 53.23 For \(S_\Co = (11,11,1)\) the long-root subdiagram is the three-legged tree \(T_{2,3,7}\), and \(\Sym\Gamma_r\) is trivial, so Theorem 53.21 presents the automorphism group of the K3 cover of an unnodal Coble surface as an extension of the Coxeter group \(W(T_{2,3,7})\) by \(\generators{\iota}\times\Aut'(Y, B)\). The group \(W(T_{2,3,7})\) is infinite, so \(S_\Co\) is reflective but not two-reflective and the K3 cover has infinite automorphism group.

Theorem 53.24 (Elliptic fibrations and isotropic vectors) Let \(X\) be a K3 surface with Picard lattice \(S_X\). The primitive isotropic vectors \(v\in S_X\) correspond bijectively to the elliptic fibrations \(X\to\PP^1\), a fibration having \(v\) as the class of its fiber.

Proof. This is (Huybrechts 2016, sec. 11.1); the correspondence in the form used here is (Alexeev 2022, sec. 1).

53.6 The ample cone

Definition 53.25 (Ample cone) Let \(X\) be a projective K3 surface with Picard lattice \(S_X\). The ample cone \(A(X)\iscontainedin S_X\tensor_\ZZ\RR\) is the set of ample classes. It is one connected component of \[ \theset{ x\in S_X\tensor_\ZZ\RR \;\middle|\; x^2 > 0,\ x\cdot d > 0 \text{ for every root } d \text{ of } S_X } , \] that is, one chamber of the complement of the mirrors of the roots inside the positive cone, in the sense of Definition 49.13.

Remark 53.26 The ample cone is a fundamental domain for \(W_2(S_X)\) acting on the positive cone, which is why the Torelli theorem presents \(\Aut(X)\) through the quotient \(\Orth^+(S_X)/W_2(S_X)\): an isometry preserving the ample cone is realized by an automorphism, and every isometry becomes one after composition with a reflection. This is the mechanism in the proof of Theorem 53.21, and the walls of \(A(X)\) are the mirrors indexed by the vertices of the Coxeter diagram \(\Gamma_2\) of Definition 53.2.