Khovanov homology of alternating links and SU(2) representations of the link group

Abstract

We prove that the Khovanov homology of alternating knots and 2-component links is equal (as a singly graded group) to the singular homology of a certain space of trace- free, binary dihedral representations of the link group. More generally, it was suggested by Kronheimer and Mrowka that the Khovanov homology of any knot might be related via gauge theory to the space of all trace-free SU(2) representations. Our result suggests that when the knot is alternating, Khovanov homology only sees the trace-free representations which are binary dihedral. The proof we give is completely elementary, using the skein exact sequence, so it does not explain why Khovanov homology should have this topological significance. In addition, we prove a conjecture of Shumakovitch that the Khovanov homology of alternating knots and 2-component links contains no n-torsion for n > 2. We also point out a relationship between the grading in Khovanov homology and the Casson-Walker invariant of the branched double cover.