Scully's generalized separation conjecture for quadratic forms

Let FF be an arbitrary field, let pp and qq be anisotropic quadratic forms over FF of dimensions at least 22, and let ss be the unique integer such that

2s<dimp2s+1.2^s<\dim p\leq 2^{s+1}.

Let F(p)F(p) be the function field of the integral affine quadric defined by p=0p=0, let i0(qF(p))\mathfrak{i}_0(q_{F(p)}) be the isotropy index of qq over F(p)F(p), and set

k=dimq2i0(qF(p)).k=\dim q-2\mathfrak{i}_0(q_{F(p)}).

Scully's conjecture. If qF(p)q_{F(p)} is isotropic, then

dimq=a2s+1+ϵ\dim q=a2^{s+1}+\epsilon

for some positive integer aa and integer ϵ[k,k]\epsilon\in[-k,k] such that ϵk(mod2)\epsilon\equiv k\pmod{2}. This conjecturally strengthens the separation theorem by constraining the dimension of a form that becomes isotropic over the function field of another quadric. The stated result has since been proved: Hoffmann established it in characteristic different from 22, and Hoffmann and Laghribi extended it to characteristic 22.

Sources & referencesView supporting material

Primary source

Stephen Scully, “Extended Karpenko and Karpenko-Merkurjev theorems for quasilinear quadratic forms”, arXiv:2409.02059 (2024).

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