Algebraic Braids and Transcendental Retractions


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September 29, 2020 3:00 pm - 4:00 pm
via Zoom Video Conferencing

Minh-Tam Trinh - MIT

If a complex, integral, projective curve C has only planar singularities, then its Jacobian admits a natural compactification with interesting topology. Work of Oblomkov, Shende, and others suggests the existence of a variety, stratified by algebraic tori and defined solely in terms of the topology of C, that retracts transcendentally onto this compactified Jacobian. We expect this retraction to be a new type of nonabelian Hodge correspondence: in particular, at the level of cohomology, it should map a (halved) weight filtration onto a filtration defined via perverse sheaves. I will construct a candidate for the larger variety using the combinatorics of braids and flag varieties, related to but ultimately different from a construction of Shende-Treumann-Zaslow. I will present evidence that the entire story is the SL_n case of a recipe that works for any semisimple group G, and that in a precise sense, these retractions should respect the isomorphism between the unipotent locus of G and the nilpotent locus of Lie(G). The key ingredient is a map from elements of the loop Lie algebra to conjugacy classes in a generalized braid group. The latter are the "algebraic braids" of the title.