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The derived deformation theory of a point

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Abstract

We provide a prorepresenting object for the noncommutative derived deformation problem of deforming a module X over a differential graded algebra. Roughly, we show that the corresponding deformation functor is homotopy prorepresented by the dual bar construction on the derived endomorphism algebra of X. We specialise to the case when X is one-dimensional over the base field, and introduce the notion of framed deformations, which rigidify the problem slightly and allow us to obtain derived analogues of the results of Ed Segal’s thesis. Our main technical tool is Koszul duality, following Pridham and Lurie’s interpretation of derived deformation theory. Along the way we prove that a large class of dgas are quasi-isomorphic to their Koszul double dual, which we interpret as a derived completion functor; this improves a theorem of Lu–Palmieri–Wu–Zhang. We also adapt our results to the setting of multi-pointed deformation theory, and furthermore give an analysis of universal prodeformations. As an application, we give a deformation-theoretic interpretation to Braun–Chuang–Lazarev’s derived quotient.

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Notes

  1. The connective hypothesis is to avoid stacky pathologies: loosely, one thinks of the connective direction of some derived-geometric object as recording derived phenomena, and one thinks of the nonconnective direction as recording stacky phenomena. More explicitly, our results fail badly if one allows nonconnective dgas as input; see 5.3.4 for an example.

  2. See [31, Chapter V] or [73, 1.23] for the definition of homotopy quotients.

  3. Composition is given by the Alexander–Whitney map.

  4. Composition is now given by the Eilenberg–Zilber map.

  5. A priori, this notation is abusive as it depends on both the choice of the cofibrant resolution of X and on the choice of lift of \(g^\circ \). However, we will see that up to quasi-isomorphism these choices do not matter.

  6. Such flopping contractions are called simple.

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  1. Departement Wiskunde-Informatica, Universiteit Antwerpen, Campus Middelheim, Middelheimlaan 1, 2020, Antwerpen, Belgium

    Matt Booth

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Booth, M. The derived deformation theory of a point. Math. Z. 300, 3023–3082 (2022). https://doi.org/10.1007/s00209-021-02892-7

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