A family of instanton-invariants for four-manifolds and their relation to Khovanov homology

Abstract: This article reviews Witten's gauge-theoretic approach to Khovanov homology from the perspective of Haydys-Witten instanton Floer theory. Expanding on Witten's arguments, we introduce a one-parameter family of instanton Floer homology groups $HF_{\theta}(W^4)$, which, based on physical arguments, are expected to be topological invariants of the four-manifold $W^4$. In analogy to the original Yang-Mills instanton Floer theory, these groups are defined by the solutions of the $\theta$-Kapustin-Witten equations on $W^4$ modulo instanton solutions of the Haydys-Witten equations that interpolate between them on the five-dimensional cylinder $\mathbb{R}s \times W^4$. The relation to knot invariants arises when the four-manifold is the geometric blowup $W^4 = [X^3 \times \mathbb{R}^+, K]$ along a knot $K \subset X^3 \times {0}$ embedded in its three-dimensional boundary. The boundaries and corners of this manifold require the specification of boundary conditions that preserve the topological invariance of the construction and are fundamentally linked to various dimensional reductions of the Haydys-Witten equations. We provide a comprehensive discussion of these dimensional reductions and relate them to well-known gauge-theoretic equations in lower dimensions, including the $\theta$-Kapustin-Witten equations, twisted extended Bogomolny equations, and twisted octonionic Nahm equations. Along the way, we record novel results on the elliptic regularity of the Haydys-Witten equations with twisted Nahm pole boundary conditions. The upshot of the article is a tentative definition of Haydys-Witten Floer theory and a precise restatement of Witten's conjecture: an equality between the Haydys-Witten Floer homology $HF^\bullet{\pi/2}([S^3 \times \mathbb{R}^+, K])$ and Khovanov homology $Kh^\bullet(K)$.