The 2-knot branched-cover conjecture for odd Khovanov homology

Let KS4K\subset S^4 be a smooth 2-knot. Removing a small neighborhood of a point of KK gives a smooth link cobordism from the empty link to the unknot UU, and hence a map

Φ ⁣:ZΛ(U)=Z{1,U}.\Phi\colon\mathbb{Z}\longrightarrow\Lambda^*(U)=\mathbb{Z}\{1,U\}.

For degree reasons, write

Φ(1)=n1\Phi(1)=n1

and define n(K)Nn(K)\in\mathbb{N} to be the absolute value of the integer nn. Let S4~K\widetilde{S^4}_K denote the branched double-cover of S4S^4, branched along KK. The 2-knot branched-cover conjecture. For every smooth 2-knot KS4K\subset S^4, the number n(K)n(K) is equal to the order of the first homology of S4~K\widetilde{S^4}_K:

n(K)=H1(S4~K;Z).n(K)=\left|H_1(\widetilde{S^4}_K;\mathbb{Z})\right|.

The invariant n(K)n(K) is intended to detect smooth 2-knots: unlike the corresponding even Khovanov invariant, which is always 11, it can be any positive odd number. The conjectured relation with the branched double-cover remains open in the supplied text.

Sources & referencesView supporting material

Primary source

Jacob Migdail and Stephan Wehrli, “Functoriality of Odd and Generalized Khovanov Homology in R^3I”, arXiv:2410.23455 (2025).

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