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35  Lattice Theory

Remark 35.1

We refer to:

35.1 Basic Theory

Remark 35.2 The study of semitoroidal compactifications of moduli spaces of Coble surfaces largely reduces to lattice theory, of which we will now recall the essential notions.

Remark 35.3 (Basic invariants) By a lattice, we mean a free \(\ZZ\)-module \(L\) of finite rank equipped with a nondegenerate1 symmetric integral bilinear form \(\beta_L: L \tensor_\ZZ L \to \ZZ\). We abbreviate \(vw \definedas \beta_L(v, w)\) and \(v^2 \definedas \beta_L(v, v)\) and refer to the latter as the norm of \(v\). We write \(L_R \definedas L\tensor_{\ZZ} R\) and \(\beta_{L_R}\) for \(R = \QQ, \RR, \CC\) for the \(\ZZ\)-linear extensions of \((L, \beta_L)\) to the rational, real, and complex numbers respectively.

A submodule \(M\iscontainedin L\) is a sublattice if the restricted bilinear form \(\restrictionof{\beta_L}{M}\) endows \(M\) with the structure of a lattice. A vector \(v\in L\) is isotropic if \(v^2 = 0\), i.e. it is norm zero, and more generally a sublattice \(M \iscontainedin L\) is isotropic if \(\restrictionof{\beta_L}{M} \equiv 0\). A lattice is said to be even if \(x^2\in 2\ZZ\) for all \(x\in L\), and odd otherwise. A nondegenerate symmetric bilinear form can be linearly extended to \(L_\RR\) and by Sylvester’s theorem, diagonalized with only \(1\) or \(-1\) on the diagonal. We write \(n_+\) and \(n_-\) respectively for the number of \(\pm 1\) entries on the diagonal. The signature of \(L\) is the pair \((n_+, n_-)\), and the index is \(n_+ - n_-\). We say \(L\) is definite if either \(n_+\) or \(n_-\) is zero, and indefinite otherwise. More precisely, if \(n_- = 0\) we say \(L\) is positive definite, and similarly if \(n_+ = 0\), we say \(L\) is negative definite. A negative definite lattice generated by elements of norm \(-2\) is said to be a root lattice, and such elements are referred to as roots.

The rank \(r\) of a lattice is its rank as a free \(\ZZ\)-module and is given by \(r=n_+ + n_- = \dim_\QQ L_\QQ\). An indefinite lattice of signature \((1, r-1)\) is said to be hyperbolic. Fixing a generating set \(e_i\) of \(L\), we define the Gram matrix of \(L\) as the matrix \(G_L \definedas (\beta_L(e_i, e_j))_{ij}\), and the discriminant as \(\disc L \definedas \det G_L\). The discriminant is independent of the choice of generating set.

Remark 35.4 (Finiteness in the definite and indefinite cases) Write \(L[k] \definedas \theset{v\in L \mid v^2 = k}\) for the set of vectors of norm \(k\).

If \(L\) is definite then \(L[k]\) is finite for every \(k\), since the real extension of \(\pm\beta_L\) is a positive definite quadratic form on \(L_\RR\) and \(L\) is discrete, so a bounded region contains finitely many lattice points. For the same reason \(\Orth(L)\) is finite: an isometry is determined by the images of a basis \(e_1,\dots,e_r\), and it carries \(e_i\) into the finite set \(L[e_i^2]\).

Neither statement survives indefiniteness. In \(U = \ZZ e\oplus\ZZ f\) one has \(U[0] = \ZZ e\union\ZZ f\), which is infinite. In \(L = \generators{1}\oplus\generators{-2}\) the set \(L[1]\) is the solution set of the Pell equation \(x^2 - 2y^2 = 1\), again infinite, and \[ M = \begin{bmatrix}3&4\\2&3\end{bmatrix} \qquad\text{satisfies}\qquad M^{\mathsf T} G_L M = G_L, \qquad G_L = \begin{bmatrix}1&0\\0&-2\end{bmatrix} , \] with \(M\) of infinite order, so \(\Orth(L)\) is infinite. Enumerating a level set or an orthogonal group is therefore not by itself an available operation for the hyperbolic lattices of this monograph, and the reflection groups of the Compactifications sections are described instead by a fundamental chamber with finitely many walls.

Remark 35.5 (Discriminant forms) The dual lattice to \(L\) is denoted \(\dualof{L} \definedas \Hom_\ZZ(L, \ZZ)\), and there is an morphism \[ \begin{aligned} \iota: L &\injects \dualof{L} \\ x &\mapsto \beta_L(x, \cdot) \end{aligned} \] which, if \(L\) is nondegenerate, is an injection with finite index image. The discriminant group is \(A_L \definedas \coker \iota \cong \dualof{L}/L\); this is a finite order group of order \(\abs{\disc L}\). We say \(L\) is unimodular if any of the following equivalent conditions hold:

  1. \(A_L\) is the trivial group,

  2. \(\iota\) is an isomorphism and \(L\cong \dualof{L}\),

  3. \(\abs{ \disc L} = 1\).

If \(A_L \cong (\zpz)^a\) for some \(a\), we say \(L\) is \(p\)-elementary; in our applications we will often have \(p=2\). For even lattices, the form \(\beta_L\) descends to a well-defined quadratic form \[ \begin{aligned} q_L: A_L &\to \QQ/2\ZZ \\ x + L &\mapsto \beta_{L_\QQ}(x, x) \mod 2\ZZ \end{aligned} \] We call the pair \((A_L, q_L)\) the discriminant quadratic form of \(L\). A morphism between two lattices is a morphism of \(\ZZ\)-modules \(\eta: L\to L'\) respecting the bilinear forms in the sense that \(\beta_L(x, y) = \beta_{L'}(\eta(x), \eta(y))\), and is a primitive embedding if \(\eta\) is injective and \(\coker \eta\) is torsionfree. An isometry of lattices is an isomorphism, defined in the obvious way. We write \(\Orth(L)\) for the group of lattice automorphisms of \(L\), denoted the orthogonal group of \(L\), and similarly \(\Orth(q_L)\) for the \(\ZZ\)-module automorphisms of the discriminant group \(A_L\) which preserve the quadratic form. There is a natural group homomorphism \(\Orth(L)\to\Orth(q_L)\), the kernel is denoted \(\tilde \Orth(L)\), equivalently written \(\Orth(L)^*\); we call it the stable orthogonal group. When \(L\) has signature \((2, n)\), the associated period domain \(\Omega_L\) has two connected components interchanged by complex conjugation, and we write \(\Orth^+(L)\) for the index-two subgroup of \(\Orth(L)\) preserving each component (equivalently, preserving the orientation of the positive-definite part, i.e. those isometries of real spinor norm \(+1\)). We write \(\Orth^+(L)^* \definedas \Orth^+(L)\intersect \tilde\Orth(L)\) for the intersection, which is the arithmetic group acting on a single component \(D_L\).

Definition 35.6 (The spinor norm and the special orthogonal group) Let \((L,\beta_L)\) be a lattice. Every \(g\in\Orth(L_\RR)\) is a product of reflections \[ g=s_{w_1}\cdots s_{w_m} \] in anisotropic vectors \(w_i\in L_\RR\), where \[ s_w(x)=x-2\frac{\beta_L(x,w)}{\beta_L(w,w)}w. \] The real spinor norm is \[ \spinornorm_\RR(g) \definedas \prod_{i=1}^m\frac{-\beta_L(w_i,w_i)}2 \in \unitsof{\RR}/(\unitsof{\RR})^2 \cong\theset{\pm1}, \] independent of the chosen reflection decomposition. The special orthogonal group is \[ \SO(L)\definedas\ker\qty{\det\colon\Orth(L)\too\theset{\pm1}}. \] For a lattice of signature \((2,n)\), \(\Orth^+(L)\) denotes the subgroup preserving either connected component of \(\Omega_L\); with the convention above this is the trivial-real-spinor-norm subgroup.

Definition 35.7 (K3 families, markings, periods, and monodromy) Let \[ \lkt\definedas U^3\oplus E_8^2 \] with its integral symmetric bilinear form; this is the K3 lattice of signature \((3,19)\).

For a connected complex manifold \(S\), let \(F_{\Kthree}(S)\) be the groupoid of smooth proper holomorphic maps \[ f\colon\mathcal X\too S \] whose fibers \(X_s\) are K3 surfaces, with morphisms the cartesian isomorphisms over \(S\). No polarization, marking, or level structure is part of an object of \(F_{\Kthree}\).

For \(f\in F_{\Kthree}(S)\), set \[ \mathbb V_f\definedas R^2f_*\underline{\ZZ}_{\mathcal X}. \] Cup product gives a morphism of integral local systems \[ \beta_f\colon \mathbb V_f\tensor_{\underline{\ZZ}_S}\mathbb V_f \too \underline{\ZZ}_S, \] whose fiber at \(s\in S\) is the K3 lattice \[ \qty{H^2(X_s,\ZZ),\smile}. \] The weight-two Hodge filtration has \[ F^2\qty{\mathbb V_f\tensor_\ZZ\mathcal O_S} = f_*\Omega^2_{\mathcal X/S}, \] a line bundle whose fiber is \(H^{2,0}(X_s)=\CC\omega_s\).

The marked K3 functor \(F_{\Kthree}^{\mathrm{marked}}(S)\) has objects \((f,\eta)\) with \(f\in F_{\Kthree}(S)\) and \[ \eta\colon \qty{\mathbb V_f,\beta_f} \iso \qty{\underline{\lkt}_S,\beta_{\lkt}) \] an isometry of local systems. Equivalently, \(\eta\) is a compatible family of fiber markings \[ \eta_s\colon \qty{H^2(X_s,\ZZ),\smile} \iso \lkt. \] A global marking trivializes the integral local system; monodromy below is therefore defined first for an arbitrary object of \(F_{\Kthree}\), using only a marking of one chosen fiber.

For any lattice \(L\), write \[ \Omega_L \definedas \theset{ [\omega]\in\PP(L_\CC) \st (\omega,\omega)=0,\ (\omega,\bar\omega)>0 }. \] If \((f,\eta)\in F_{\Kthree}^{\mathrm{marked}}(S)\), its period map is \[ \operatorname{per}_{f,\eta}\colon S\too\Omega_{\lkt}, \qquad s\longmapsto [\eta_{s,\CC}(\omega_s)], \] where \(0\neq\omega_s\in H^{2,0}(X_s)\). The notation \([\omega]\) always denotes the projective class of a nonzero holomorphic symplectic form after the relevant marking.

Now let \(f\in F_{\Kthree}(S)\), choose \(s_0\in S\), and choose a marking of the single fiber \[ \eta_{s_0}\colon H^2(X_{s_0},\ZZ)\iso\lkt. \] The local system \(\mathbb V_f\) has monodromy representation \[ \rho_{f,s_0}\colon \pi_1(S,s_0) \too \Orth\qty{H^2(X_{s_0},\ZZ),\smile}. \] Transporting through \(\eta_{s_0}\) gives \[ \rho_{f,s_0}^{\eta_{s_0}} \definedas \eta_{s_0}\rho_{f,s_0}\eta_{s_0}^{-1} \colon \pi_1(S,s_0) \too \Orth(\lkt). \] Define the monodromy group of the marked base fiber of the family by \[ \operatorname{Mon}(f,s_0,\eta_{s_0}) \definedas \im\rho_{f,s_0}^{\eta_{s_0}} \iscontainedin \Orth(\lkt). \] Thus monodromy is attached to the local system \(\mathbb V_f\); the marking only identifies its fiber with the fixed lattice \(\lkt\).

Finally, define \[ \operatorname{Mon}(\lkt) \] to be the subgroup of \(\Orth(\lkt)\) generated by the images \(\operatorname{Mon}(f,s_0,\eta_{s_0})\) as \(f\), \(S\), \(s_0\), and the base-fiber marking vary.

Definition 35.8 (The orientation character of the K3 lattice) Set \[ \widetilde C_{\lkt} \definedas \theset{x\in\lkt_\RR\st(x,x)>0}. \] For a positive-definite three-plane \(W_3\containedin\lkt_\RR\), the inclusion \[ W_3\sm\theset{0}\injects\widetilde C_{\lkt} \] is a homotopy equivalence: in the orthogonal decomposition \[ \lkt_\RR=W_3\oplus W_3^\perp, \] scaling the \(W_3^\perp\)-component to zero preserves positivity. Hence \[ H_2\qty{W_3\sm\theset{0},\ZZ} \iso H_2\qty{\widetilde C_{\lkt},\ZZ} \iso \ZZ. \] An orientation of \(W_3\) determines one of the two generators; these are the orientation classes of \(\widetilde C_{\lkt}\). The action on orientation classes defines \[ \sigma_{\lkt}\colon \Orth(\lkt_\RR) \too \theset{\pm1}. \] This is the positive-three-plane form of the real spinor-norm character; set \[ \Orth^+(\lkt) \definedas \ker\sigma_{\lkt}. \] This is the convention used for the K3 lattice in Huybrechts, Lectures on K3 Surfaces, §5.4, and is the orientation-class formulation used in Markman, A survey of Torelli and monodromy results for holomorphic-symplectic varieties, §4.

Proposition 35.9 (K3 monodromy preserves the positive-three-plane orientation) Let \(f\in F_{\Kthree}(S)\) with \(S\) connected, let \(s_0\in S\), and choose a fiber marking \[ \eta_{s_0}\colon H^2(X_{s_0},\ZZ)\iso\lkt. \] Then \[ \operatorname{Mon}(f,s_0,\eta_{s_0}) \iscontainedin \Orth^+(\lkt). \] Consequently \[ \operatorname{Mon}(\lkt) \iscontainedin \Orth^+(\lkt). \]

Proof. For each \(s\in S\), choose \(0\neq\omega_s\in H^{2,0}(X_s)\) and a Kähler class \[ H_s\in H^{1,1}(X_s,\RR). \] The real three-plane \[ W_s \definedas \Span_\RR\qty{\Re\omega_s,\Im\omega_s,H_s} \containedin H^2(X_s,\RR) \] is positive definite for the cup-product form. The ordered basis \[ \qty{\Re\omega_s,\Im\omega_s,H_s} \] defines an orientation class of the positive cone of \(H^2(X_s,\RR)\).

This class is independent of the choices: replacing \(\omega_s\) by \(a\omega_s\) with \(a\in\CC^*\) acts orientation-preservingly on the real plane \(H^{2,0}(X_s)\oplus H^{0,2}(X_s)\), and the Kähler cone is connected. In a local trivialization of \(\mathbb V_f\), the Hodge line and a Kähler class vary continuously, so these orientation classes form a locally constant section of the two-element orientation local system associated to the positive cones.

Let \(\gamma\in\pi_1(S,s_0)\). Gauss–Manin parallel transport along \(\gamma\) therefore preserves the distinguished orientation class at \(s_0\). After applying \(\eta_{s_0}\), this says \[ \sigma_{\lkt}\qty{\rho_{f,s_0}^{\eta_{s_0}}(\gamma)}=+1. \] Hence \[ \im\rho_{f,s_0}^{\eta_{s_0}} \iscontainedin \ker\sigma_{\lkt} = \Orth^+(\lkt). \] Taking the subgroup generated by all such family monodromy images gives the second containment.

Definition 35.10 (Degree-$2d$ K3 families and the primitive period lattice) Fix a primitive vector \[ h\in\lkt, \qquad h^2=2d, \] and set \[ \lkttd \definedas \generators{h}^{\perp\lkt} \cong \generators{-2d}\oplus U^2\oplus E_8^2. \] Thus \(\lkttd\) is the degree-\(2d\) primitive K3 lattice already used for \(\ftd\).

For a connected complex manifold \(S\), an object of \(\ftd(S)\) is a pair \[ \qty{f\colon\mathcal X\too S,\mathcal L} \] with \(f\in F_{\Kthree}(S)\) and \(\mathcal L\) a relatively ample primitive line bundle satisfying \[ c_1(\mathcal L_s)^2=2d \] on every fiber. The classes \(c_1(\mathcal L_s)\) form a flat section \[ c_1(\mathcal L)\in\Gamma(S,\mathbb V_f), \] so the primitive local system \[ \mathbb V_{f,\mathcal L}^{\mathrm{prim}} \definedas c_1(\mathcal L)^\perp \iscontainedin \mathbb V_f \] is monodromy invariant.

A marking of the polarized family is a marking \[ \eta\colon\mathbb V_f\iso\underline{\lkt}_S \] such that \[ \eta\qty{c_1(\mathcal L)}=h. \] It restricts to \[ \eta^{\mathrm{prim}}\colon \mathbb V_{f,\mathcal L}^{\mathrm{prim}} \iso \underline{\lkttd}_S. \] The marked polarized period map is therefore \[ \operatorname{per}_{f,\mathcal L,\eta} \colon S\too\Omega_{\lkttd}, \qquad s\longmapsto [\eta_{s,\CC}(\omega_s)]. \]

For an unmarked polarized family, the monodromy representation of \(\mathbb V_{f,\mathcal L}^{\mathrm{prim}}\) lands, after a base-fiber marking carrying \(c_1(\mathcal L_{s_0})\) to \(h\), in \[ \Orth^+(\lkttd), \] equivalently in the subgroup preserving a connected component of the type-IV domain \(\Omega_{\lkttd}\). This is the polarized form of the positive-orientation statement above.

Proposition 35.11 (Polarized K3 monodromy preserves the primitive period component) Let \((f\colon\mathcal X\to S,\mathcal L)\in \ftd(S)\) with \(S\) connected, choose \(s_0\in S\), and choose a fiber marking \[ \eta_{s_0}\colon H^2(X_{s_0},\ZZ)\iso\lkt \] with \[ \eta_{s_0}\qty{c_1(\mathcal L_{s_0})}=h. \] Then the monodromy representation of the primitive local system \[ \mathbb V_{f,\mathcal L}^{\mathrm{prim}} \] lands, after the induced identification with \(\lkttd=h^{\perp\lkt}\), in \[ \Orth^+(\lkttd). \]

Proof. Because \(c_1(\mathcal L)\) is a flat section of \(\mathbb V_f\), every element \[ g\in\operatorname{Mon}(f,s_0,\eta_{s_0}) \iscontainedin\Orth(\lkt) \] fixes \(h\). By Proposition 35.9, \(g\in\Orth^+(\lkt)\). Let \[ P\containedin\lkttd_\RR \] be an oriented positive two-plane. Since \(h^2>0\) and \(P\perp h\), the direct sum \[ \RR h\oplus P\containedin\lkt_\RR \] is a positive three-plane. Orient it by the positive vector \(h\) followed by an oriented basis of \(P\). Because \(g(h)=h\), preservation by \(g\) of the orientation of \(\RR h\oplus P\) is equivalent to preservation by \(g\vert_{\lkttd}\) of the orientation of \(P\). Hence \[ g\vert_{\lkttd}\in\Orth^+(\lkttd), \] which is equivalently the subgroup preserving a connected component of \(\Omega_{\lkttd}\).

Remark 35.12 (Orthogonal complements) Given two lattices \(L_1, L_2\) we write \(L_1\oplus L_2\) for the orthogonal direct sum, which is the direct sum of the underlying modules with bilinear form defined by \[ \beta_{L_1\oplus L_2}(v_1 + v_2, w_1 + w_2)\definedas \beta_{L_1}(v_1, w_1) + \beta_{L_2}(v_2, w_2) . \] We write \(L^{\oplus n}\) for the direct sum of \(n\) copies of \(L\). Let \(\eta: M \injects L\) be a primitive embedding of lattices, for example the inclusion of a sublattice. We write \[ M^{\perp L} \definedas \theset{x\in L \mid \beta_L(x, M) = 0} . \] If the ambient lattice \(L\) is understood, we often abuse notation and simply write \(M^{\perp}\) without reference to \(L\). Note that \(M^{\perp L}\oplus M \iscontainedin L\) may not be saturated, and is generally a finite index sublattice of \(L\). We note that \(M^{\perp L}\intersect M \neq \theset{0}\) in general.2 We also note that for any lattices \(L_i\), \[ A_{L_1 \oplus \cdots \oplus L_n} = A_{L_1} \oplus \cdots \oplus A_{L_n} \]

Remark 35.13 (2-elementary lattices) Let \(L\) be a 2-elementary lattice. The divisibility of a vector \(v\in L\), denoted \(\div_L(v)\), is defined by \(\beta_L(v, L) = \div_L(v)\ZZ\), i.e. the positive integral generator of the image of the map \(\beta_L(v, \cdot): L\to \ZZ\). For 2-elementary lattices, one always has \(\div_L(v) \in \theset{1, 2}\). We set \(v^* \definedas v/\div_L(v)\in A_L\). Letting \(q_L:A_L \to {1\over 2}\ZZ/\ZZ\) be the induced quadratic form on \(A_L\), we say \(v^*\) is characteristic if \(q_L(x) = \beta_L(v^*, x)\mod \ZZ\) for all \(x\in A_L\), and is ordinary otherwise. We say that a primitive isotropic vector \(e\in L\) is

  1. odd if \(\div_L(e) = 1\),

  2. even ordinary if \(\div_L(e) = 2\) and \(e^*\) is ordinary, or

  3. even characteristic if \(\div_L(e) = 2\) and \(e^*\) is characteristic.

The 2-elementary hyperbolic lattices admitting a primitive embedding into \(\lkt\) were classified by Nikulin in (Nikulin 1980, sec. 3.6.2). An indefinite even 2-elementary lattice is determined up to isometry by its signature together with a triple of invariants \((r,a,\delta)\). Here, \(r\definedas \rank_\ZZ(L)\) is the rank, \(a = \rank_{\bF_2}A_L\) is the exponent appearing in \(A_L = (\ZZ/2\ZZ)^a\), and \(\delta \in \theset{0, 1}\) is the coparity: we set \(\delta = 0\) if \(q_L(A_L) \iscontainedin \ZZ\), so \(q_L(x) \equiv 0 \mod \ZZ\) for all \(x\in A_L\), and \(\delta=1\) otherwise. The evenness and indefiniteness hypotheses are both needed: the theorem is Nikulin’s classification of even 2-elementary lattices, and it rests on the general uniqueness criterion for even indefinite lattices (Nikulin 1980 Thm. 1.14.2). We accordingly specify such lattices using the notation \((r,a,\delta)_{n_+}\).

Theorem 35.14 (Classification of primitive isotropic vectors by type) Let \(S\) be an even hyperbolic 2-elementary lattice with invariants \((r, a, \delta)\), let \(v\in S\) be a primitive isotropic vector, and write \(\oS\definedas v^{\perp S}/v\). Then exactly one of the following holds, according to the type of \(v\) in the sense above, and in each case \(S\) splits off the indicated rank-two summand containing \(v\):

  1. odd: \(S \cong U\oplus\oS\), with \(a_{\oS} = a\) and \(\delta_{\oS} = \delta\);

  2. even ordinary: \(S \cong U(2)\oplus\oS\), with \(a_{\oS} = a - 2\) and \(\delta_{\oS} = \delta\);

  3. even characteristic: \(S \cong \latI_{1,1}(2)\oplus\oS\), with \(a_{\oS} = a - 2\); this case forces \(\delta = 1\) and gives \(\delta_{\oS} = 0\).

In all three cases \(\oS\) is even negative definite of rank \(r - 2\). Classifying the primitive isotropic vectors of \(S\) up to isometry is therefore equivalent to classifying the even negative definite lattices with invariants \((r-2, a, \delta)\), \((r-2, a-2, \delta)\) and \((r-2, a-2, 0)\).

Proof. This is (Alexeev and Engel 2022 Prop. 5.5).

Remark 35.15 The isotropic-trichotomy theorem (Theorem 35.14) is the tool by which a \(0\)-cusp of a type IV quotient is identified from a single numerical invariant of its isotropic vector: the divisibility, together with the characteristic-or-ordinary dichotomy, determines the invariants of the boundary lattice \(\overline{S}\), and for indefinite \(\overline{S}\) those invariants determine its isometry class. It is used in this form throughout the cusp correspondence.

Remark 35.16 (Twists of a lattice) If \(L\) is a lattice with bilinear form \(\beta_L\), define \(L(n)\) to be the twist of \(L\) by \(n\), which has the same underlying \(\ZZ\)-module but is equipped with the scaled bilinear form \[ \beta_{L(n)}(v,w) \definedas n\cdot \beta_L(v, w). \]

Remark 35.17 (The lattice $\generators{n}$) The lattice \(\generators{n}\) is defined as the rank 1 lattice \(\ZZ\) with one generator \(v\) satisfying \(\beta_{\generators{n}}(v,v) = n\). The Gram matrix is the \(1\times 1\) matrix \(G_{\generators n} = [n]\), and the associated quadratic form is \(q_{\generators{n}}(x) = nx^2\).

Remark 35.18 (The hyperbolic lattice) In rank 2, there are two unimodular hyperbolic lattices: the odd \(\latI_{1, 1} \definedas \generators{1} \oplus \generators{-1}\), and the even \(U\definedas \latII_{1, 1}\). We refer to the latter as the hyperbolic lattice, which can be realized as \(U \definedas \ZZ e \oplus \ZZ f\) with \(e^2=f^2 = 0\) and \(ef = 1\), and thus the following Gram matrix: \[ G_U = \begin{bmatrix}0&1\\1&0\end{bmatrix} . \]

Remark 35.19 (ADE lattices) Any Dynkin diagram of type \(A_n, D_n, E_6, E_7, E_8\) corresponds to a root lattice of the respective type. By convention, we take the negative definite twists of these lattices. Of particular importance to us is the \(E_8\) lattice associated to the following Dynkin diagram:

Figure 35.1: The Dynkin diagram \(E_{8}\).

Remark 35.20 (The lattice $\latI_{p, q}$) For any pair of non-negative integers \((p, q)\), there exists an odd indefinite unimodular lattice determined up to isomorphism by its rank and signature: \[ \latI_{p, q} \definedas \generators{1}^{\oplus p}\oplus \generators{-1}^{\oplus q} . \]

Remark 35.21 (The lattice $\latII_{p, q}$) Let \(L\) be an even indefinite unimodular lattice of signature \((p, q)\). Then \(p-q\equiv 0 \pmod 8\), and \(L\) is uniquely determined up to isomorphism by its rank and signature: \[ \latII_{p, q} \definedas \begin{cases} U ^{\oplus p}\oplus E_8^{\oplus {q-p\over 8}}, & p < q \\ U^{\oplus q} \oplus E_8(-1)^{\oplus {p-q\over 8}}, & p > q .\end{cases} \]


  1. A bilinear form is nondegenerate if for any \(x\), \(\beta_L(x, L) = 0\) implies \(x=0\).↩︎

  2. For example, let \(L = U = \generators{e,f}\) and \(M = \generators{e}\). Then \(M^{\perp L} = M\).↩︎