The integrality and zero-block conjecture for quadratic lattice extensions

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Let L=M⊕NL=M\oplus N be a quadratic lattice, where MM is spanned by (e1,…,en−d)(e_1,\ldots,e_{n-d}) and NN by (en−d+1,…,en)(e_{n-d+1},\ldots,e_n). Let L′=M′⊕NL'=M'\oplus N be a lattice containing LL in L⊗oFL\otimes_{\mathfrak{o}}F, with M′M' containing MM in M⊗oFM\otimes_{\mathfrak{o}}F. Let qL′q_{L'} and qM′q_{M'} be the quadratic forms induced from qLq_L. When qM′q_{M'} is integral, write Mˉ′=M′/πM′\bar{M}'=M'/\pi M' for its reduction modulo π\pi; when qL′q_{L'} is integral, likewise write Lˉ′=L′/πL′\bar{L}'=L'/\pi L'. Integrality and zero-block conjecture. If qM′q_{M'} on M′M' is integral, then qL′q_{L'} on L′L' is integral, and hence these conditions are equivalent. In this situation, the dimensions of Lˉ′\bar{L}' and Mˉ′\bar{M}' modulo their radicals are equal. Equivalently, the number of zero entries in GK(L′)\mathrm{GK}(L') equals the number of zero entries in GK(M′)\mathrm{GK}(M'). The conjecture would characterize the integrality of qL′q_{L'} through the summand M′M' and relate the resulting reduced quadratic spaces to the zero entries of their Gross–Keating invariants; the paper proves it when pp is odd or when LL is anisotropic over Z2\mathbb{Z}_2, while the general case remains open.

References

Primary source

Sungmun Cho and Takuya Yamauchi, “A reformulation of the Siegel series and intersection numbers”, arXiv:1805.01666 (2020).

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