Floer theory for the variation operator of an isolated singularity

Abstract The variation operator in singularity theory maps relative homology cycles to compact cycles in the Milnor fiber using the monodromy. We construct its symplectic analog for an isolated singularity. We define the monodromy Lagrangian Floer cohomology, which provides categorifications of the standard theorems on the variation operator and the Seifert form. The key ingredients are a special class in the symplectic cohomology of the inverse of the monodromy and its closed–open images. For isolated plane curve singularities whose A'Campo divide has depth zero, we find an exceptional collection consisting of noncompact Lagrangians in the Milnor fiber corresponding to a distinguished collection of vanishing cycles under the variation operator.