I am interested in the general theory of deformations locally ringed spaces in the same language of the deformation theory of schemes/varieties that is already widely available. I am aware for example of what is written down in the stacks project (for example Tag 08UX) but I'm failing to find a more through treatment specially one that would put in contrast the differences with the special case of schemes/varieties. In my literature review, however, I stumbled into the following quote in
Lowen, Wendy; van den Bergh, Michel, Deformation theory of abelian categories, Trans. Am. Math. Soc. 358, No. 12, 5441-5483 (2006). ZBL1113.13009.
These results confirm the fundamental insight of Gerstenhaber and Schack [6,
8] that one should define the deformations of a ringed space $(X, \mathcal{O}_X)$ not as the deformations of $\mathcal{O}_X$ as a sheaf of k-algebras, but rather as the deformations of the k-linear category $\mathfrak{u}$ (or of the “diagram” $(\mathcal{B}, \mathcal{O}_{\mathcal{B}})$ in case $X\in \mathcal{B}$). These “virtual” deformations are nothing but the deformations of the abelian category $Mod(\mathcal{O}_{X})$.
The cited category $\mathfrak{u}$ is an appropriately defined category which is used to show that the deformations of the category of presheaves of modules over an appropriate basis $\mathcal{B}$ are equivalent to deformations of sheaves of modules over $\mathcal{O}_{X}$
I have skimmed through the literature including the cited papers and while I have found some indication of the exact meaning of this claim I am still confused. My interpretation is that the claim is either strictly noncommutative in nature ( so passing through a reconstruction theorem ), or deformations of this abelian category directly give the 'correct' deformation theory of the space in some sense ( for example the relationship with Hochschild cohomology in Lowen, Wendy; van den Bergh, Michel, Hochschild cohomology of Abelian categories and ringed spaces, Adv. Math. 198, No. 1, 172-221 (2005). ZBL1095.13013.)
My first question would then be, could somebody clarify what exactly is the quote saying?
My second question is then, if the space is a (sufficiently nice) scheme then am I to understand that the deformation theory of the category of quasi-coherent sheaves controls the deformations of the scheme as I would find it written in classical texts?
Thanks in advance
The cited papers on the quote are, respectively:
Gerstenhaber, M.; Schack, S. D., On the deformation of algebra morphisms and diagrams, Trans. Am. Math. Soc. 279, 1-50 (1983). ZBL0544.18005.
and,
Gerstenhaber, Murray; Schack, Samuel D., The cohomology of presheaves of algebras. I: Presheaves over a partially ordered set, Trans. Am. Math. Soc. 310, No. 1, 135-165 (1988). ZBL0706.16021.
Best Answer
As Jon Pridham notes in the comments, the quote should be understood noncommutatively. In fact, in the introduction Lowen and Van den Bergh write
As you observe, this is a natural point of view when identifying a ringed space $(X, \mathcal O_X)$ with its category of quasi-coherent sheaves by considering spectra of Abelian categories as in the Gabriel–Rosenberg reconstruction theorem.
To answer your second question, the deformations of the Abelian category $\mathrm{Qcoh} (X)$ of quasi-coherent sheaves contain classical deformations of $X$ as a special case, but in general admit more deformations than $X$.
The difference becomes quite clear when looking at the space of first-order deformations up to equivalence (i.e. the tangent space of the deformation functor).
For example, for smooth $X$ one gets
For affine $X = \mathrm{Spec} (A)$ with $A$ commutative one gets
If you want to compare deformations of $\mathrm{Qcoh} (X)$ and $X$ on equal footing, you can look at deformations of $\mathcal O_X \vert_{\mathfrak U}$ for some affine open cover $\mathfrak U$ closed under $\cap$ either
The "insight" by Gerstenhaber and Schack was that the former type of deformations of the diagram $\mathcal O_X \vert_{\mathfrak U}$ give a useful generalization of "deformation" of a ringed space, and Lowen and Van den Bergh show that this corresponds to the deformation theory of $\mathrm{Qcoh} (X)$ as Abelian category.
[1] Lepri, Emma; Manetti, Marco, On deformations of diagrams of commutative algebras, Colombo, Elisabetta (ed.) et al., Birational geometry and moduli spaces. Collected papers presented at the INdAM workshop, Rome, Italy, June 11–15, 2018. Cham: Springer. Springer INdAM Ser. 39, 77-107 (2020). ZBL1440.13066.