Cancellation conjecture for nonsingular full Q-forms

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Let QQ be a form parameter, and let a nonsingular full QQ-form be a nonsingular QQ-form whose linearisation is surjective. Write HbϵQ(Zm)H_{b\epsilon_Q}(\mathbb{Z}^m) for the standard hyperbolic form on Z2m\mathbb{Z}^{2m}, and let g(μ‾)\mathrm{g}(\underline \mu) denote the genus, namely the largest non-negative integer rr such that

μ‾≅μ‾′⊕HϵQ(Zr)\underline \mu\cong\underline \mu'\oplus H_{\epsilon_Q}(\mathbb{Z}^r)

for a nonsingular full QQ-form μ‾′\underline \mu'. Cancellation conjecture. Let μ‾0\underline \mu_0 and μ‾1\underline \mu_1 be nonsingular full QQ-forms of the same rank with [μ‾0]=[μ‾1]∈W0(Q)[\underline \mu_0]=[\underline \mu_1]\in W_0(Q). Then: (a) for some non-negative integer mm, μ‾0⊕HϵQ(Zm)\underline \mu_0\oplus H_{\epsilon_Q}(\mathbb{Z}^m) is isomorphic to μ‾1⊕HϵQ(Zm)\underline \mu_1\oplus H_{\epsilon_Q}(\mathbb{Z}^m); and (b) if ϵQ=1\epsilon_Q=1 and g(μ‾0)≥1\mathrm{g}(\underline \mu_0)\geq1, or if ϵQ=−1\epsilon_Q=-1, then μ‾0\underline \mu_0 and μ‾1\underline \mu_1 are isomorphic. This is motivated by Kreck’s stable diffeomorphism and cancellation theorems and proposes algebraic analogues for full nonsingular QQ-forms; the source supplies no resolution, so the conjecture is treated as open.

References

Primary source

Diarmuid Crowley and Csaba Nagy, “The Witt groups of extended quadratic forms over Z”, arXiv:2404.09189 (2026).

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