9,794 research outputs found

    On the sheaf-theoretic SL(2, C) Casson–Lin invariant

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    We prove that the (τ-weighted, sheaf-theoretic) SL(2, C) Casson–Lin invariant introduced by Manolescu and the first author is generically independent of the parameter τ and additive under connected sums of knots in integral homology 3-spheres. This addresses two questions asked by Manolescu and the first author. Our arguments involve a mix of topology and algebraic geometry, and rely crucially on the fact that the SL(2, C) Casson–Lin invariant admits an alternative interpretation via the theory of Behrend functions.</p

    A unified Casson-Lin invariant for the real forms of SL(2)

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    We introduce a unified framework for counting representations of knot groups into SU(2)SU(2) and SL(2,R)SL(2, \mathbb{R}). For a knot KK in the 3-sphere, Lin and others showed that a Casson-style count of SU(2)SU(2) representations with fixed meridional holonomy recovers the signature function of KK. For knots whose complement contains no closed essential surface, we show there is an analogous count for SL(2,R)SL(2, \mathbb{R}) representations. We then prove the SL(2,R)SL(2, \mathbb{R}) count is determined by the SU(2)SU(2) count and a single integer h(K)h(K), allowing us to show the existence of various SL(2,R)SL(2, \mathbb{R}) representations using only elementary topological hypotheses. Combined with the translation extension locus of Culler-Dunfield, we use this to prove left-orderability of many 3-manifold groups obtained by cyclic branched covers and Dehn fillings on broad classes of knots. We give further applications to the existence of real parabolic representations, including a generalization of the Riley Conjecture (proved by Gordon) to alternating knots. These invariants exhibit some intriguing patterns that deserve explanation, and we include many open questions. The close connection between SU(2)SU(2) and SL(2,R)SL(2, \mathbb{R}) comes from viewing their representations as the real points of the appropriate SL(2,C)SL(2, \mathbb{C}) character variety. While such real loci are typically highly singular at the reducible characters that are common to both SU(2)SU(2) and SL(2,R)SL(2, \mathbb{R}), in the relevant situations, we show how to resolve these real algebraic sets into smooth manifolds. We construct these resolutions using the geometric transition S2E2H2S^2 \to \mathbb{E}^2 \to \mathbb{H}^2, studied from the perspective of projective geometry, and they allow us to pass between Casson-Lin counts of SU(2)SU(2) and SL(2,R)SL(2, \mathbb{R}) representations unimpeded.148 pages, 24 figures; v2: incorporates referee\u27s comments; to appear in Geometry and Topolog
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