Tag 0281

\(U^{3} \oplus E_{8}(-1)^{2} \oplus \langle -6 \rangle\)

U^3 + E8(-1)^2 + <-6>·Beauville-Bogomolov lattice of K3^[4]

Invariants

Rank	23
Signature \((n_+, n_-)\)	(3, 20)
Determinant	\(6\)

Gram tensor

The components \(b(e_i, e_j)\) of the Gram tensor \(b\) in the chosen basis \(e_{1}, \dots, e_{23}\).

In this basis, \(L = {}\)\(\mathbb{Z}\{e_{1}, e_{2}\}\) \(\oplus\) \(\mathbb{Z}\{e_{3}, e_{4}\}\) \(\oplus\) \(\mathbb{Z}\{e_{5}, e_{6}\}\) \(\oplus\) \(\mathbb{Z}\{e_{7}, \dots, e_{14}\}\) \(\oplus\) \(\mathbb{Z}\{e_{15}, \dots, e_{22}\}\) \(\oplus\) \(\mathbb{Z}\{e_{23}\}\), an orthogonal sum.

	\(e_{1}\)	\(e_{2}\)	\(e_{3}\)	\(e_{4}\)	\(e_{5}\)	\(e_{6}\)	\(e_{7}\)	\(e_{8}\)	\(e_{9}\)	\(e_{10}\)	\(e_{11}\)	\(e_{12}\)	\(e_{13}\)	\(e_{14}\)	\(e_{15}\)	\(e_{16}\)	\(e_{17}\)	\(e_{18}\)	\(e_{19}\)	\(e_{20}\)	\(e_{21}\)	\(e_{22}\)	\(e_{23}\)
\(e_{1}\)	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{2}\)	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{3}\)	0	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{4}\)	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{5}\)	0	0	0	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{6}\)	0	0	0	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{7}\)	0	0	0	0	0	0	-2	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{8}\)	0	0	0	0	0	0	0	-2	0	1	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{9}\)	0	0	0	0	0	0	1	0	-2	1	0	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{10}\)	0	0	0	0	0	0	0	1	1	-2	1	0	0	0	0	0	0	0	0	0	0	0	0
\(e_{11}\)	0	0	0	0	0	0	0	0	0	1	-2	1	0	0	0	0	0	0	0	0	0	0	0
\(e_{12}\)	0	0	0	0	0	0	0	0	0	0	1	-2	1	0	0	0	0	0	0	0	0	0	0
\(e_{13}\)	0	0	0	0	0	0	0	0	0	0	0	1	-2	1	0	0	0	0	0	0	0	0	0
\(e_{14}\)	0	0	0	0	0	0	0	0	0	0	0	0	1	-2	0	0	0	0	0	0	0	0	0
\(e_{15}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	-2	0	1	0	0	0	0	0	0
\(e_{16}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	-2	0	1	0	0	0	0	0
\(e_{17}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	0	-2	1	0	0	0	0	0
\(e_{18}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	1	-2	1	0	0	0	0
\(e_{19}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	-2	1	0	0	0
\(e_{20}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	-2	1	0	0
\(e_{21}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	-2	1	0
\(e_{22}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	-2	0
\(e_{23}\)	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	-6

[[0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, -2, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, -2, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 1, 0, -2, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 1, 1, -2, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -2, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -2, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -2, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -2, 0, 0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -2, 0, 1, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -2, 0, 1, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, -2, 1, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, -2, 1, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -2, 1, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -2, 1, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -2, 1, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, -2, 0], [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -6]]

Integral lattice

Parity	even
Discriminant group	\(\mathbb{Z}/6\)
Unimodular	no
Integral overlattices \(L \subseteq M \subseteq L^*\)	1
Primes of bad reduction \(\Sigma_L = \{p \mid 2 \det L\}\)	\(\{2, 3\}\)
Zeta function of the cone \(X_0 : Q(x) = 0\), \(\Sigma = \Sigma_L\)	\(\zeta^\Sigma(X_0, s) = \zeta^\Sigma(s - 22)\)
Zeta function of \(X_n : Q(x) = n\), \(n \neq 0\), \(\Sigma = \Sigma_L \cup \{p \mid n\}\)	\(\zeta^\Sigma(X_n, s) = \zeta^\Sigma(s - 22)\, L^\Sigma(s - 11, \chi_{-6n})\)
Hyperbolic index \(\max\{n : L \cong U^n \oplus L'\}\)	\(\geq 3\), by an embedding of \(U^{3}\) in the morphism files

Indefinite lattice

Isotropic	yes

Roots

Root lattice: \(L = \mathbb{Z}\Phi(L)\)	yes
\(L = \mathbb{Z}\Phi_S(L)\) for	\(S = \{-6, -2, 2\}\)
\(R(L) = \mathbb{Z}\Phi(L)\)	\(L\)
Rank of \(R(L)\)	23
\([L : R(L)]\)	1
\(R(L)\) is primitive in \(L\)	yes
Proof	Each \(r_i\) in roots that generate \(\mathbb{Z}\Phi(L)\) is a root, and the \(r_i\) generate \(L\).

Roots that generate \(\mathbb{Z}\Phi(L)\)

The \(r_i\) are a basis of \(L\).

Root	Coordinates	\(b(r, r)\)
\(r_{1}\)	\(e_{7}\)	\(-2\)
\(r_{2}\)	\(e_{8}\)	\(-2\)
\(r_{3}\)	\(e_{9}\)	\(-2\)
\(r_{4}\)	\(e_{10}\)	\(-2\)
\(r_{5}\)	\(e_{11}\)	\(-2\)
\(r_{6}\)	\(e_{12}\)	\(-2\)
\(r_{7}\)	\(e_{13}\)	\(-2\)
\(r_{8}\)	\(e_{14}\)	\(-2\)
\(r_{9}\)	\(e_{15}\)	\(-2\)
\(r_{10}\)	\(e_{16}\)	\(-2\)
\(r_{11}\)	\(e_{17}\)	\(-2\)
\(r_{12}\)	\(e_{18}\)	\(-2\)
\(r_{13}\)	\(e_{19}\)	\(-2\)
\(r_{14}\)	\(e_{20}\)	\(-2\)
\(r_{15}\)	\(e_{21}\)	\(-2\)
\(r_{16}\)	\(e_{22}\)	\(-2\)
\(r_{17}\)	\(e_{23}\)	\(-6\)
\(r_{18}\)	\(e_{1} + e_{2}\)	\(2\)
\(r_{19}\)	\(e_{1} + e_{7}\)	\(-2\)
\(r_{20}\)	\(e_{3} + e_{4}\)	\(2\)
\(r_{21}\)	\(e_{3} + e_{7}\)	\(-2\)
\(r_{22}\)	\(e_{5} + e_{6}\)	\(2\)
\(r_{23}\)	\(e_{5} + e_{7}\)	\(-2\)

Notes

\(H^2(Y, \mathbb{Z})\) with the Beauville-Bogomolov form, for \(Y\) deformation equivalent to the Hilbert scheme \(S^{[4]}\) of \(4\) points on a K3 surface \(S\): \(U^3 \oplus E_8(-1)^2 \oplus \langle -6 \rangle\) (Rapagnetta).

Morphisms

References

A. Rapagnetta, "On the Beauville form of the known irreducible symplectic varieties", Math. Ann. 340 (2008), 77-95, arXiv:math/0606409, the table of the introduction.