# Algebraic definition of the Kuranishi map

Let $X$ be a smooth projective algebraic variety over an algebraically closed field $k$. If $k=\mathbb{C}$, we know by work of Kuranishi that the base of the versal deformation of $X$ is the germ at $0$ of the fiber over $0$ of a holomorphic map $K:H^1(X, T_X)\to H^2(X, T_X)$ (defined in the neighborhood of 0), called the Kuranishi map.

This means that, if $S_i$ are the power series rings associated to $H^i(X, T_X)$ (i.e., the completions of $Sym^* H^i(X, T_X)^*$), there is a map $k: S_2 \to S_1$ such that $R = S_1 \otimes_{S_2} k$ pro-represents the deformation functor of $X$.

Question. Can one construct the map $k$ using algebraic methods?

Probably one should assume that $k$ is of characteristic zero (or replace power series rings by completed divided power algebras...).

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## 2 Answers

You can look at Manetti's paper Deformation theory via differential graded Lie algebras, arXiv:math/0507284.

As the title suggest, it follows the philosophy that every deformation problem is governed by a DGLA, via solution of Maurer-cartan equations (module gauge action).

One of the main advantages with respect to the classical Kodaira-Spencer's approach is that the theory works over any field $\mathbb{K}$ of characteristic $0$.

The construction of the Kuranishi map is presented in Section $4$.

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Many thanks! This article was already high on my reading list! – Piotr Achinger Jan 16 '13 at 0:53

In characteristic zero the answer is positive.

If $L^\bullet$ is the dgla (differential graded Lie algebra) governing your deformation problem (the Kodaira-Spencer dgla $\oplus A^{0,p}(X,T_X)$ in your example), the formal Kuranishi theorem states that for every splitting $\delta$ of $L^\bullet$, there exists a hull $Kur^\delta_{L^\bullet}\to Def_{L^\bullet}$, the formal Kuranishi space. Here $Def_{L^\bullet}:Art_{k}\to Sets$ is the deformation functor associated to $L^\bullet$. A splitting of $L^\bullet$ is a degree $-1$ linear map, $\delta:L^\bullet\to L^\bullet[-1]$, such that $\delta^2=0$, $d\delta d=d$, $\delta d\delta=\delta$. It plays the role of $d^\ast G$ in Hodge theory, where $G$ is Green's operator and $d^\ast$ is the adjoint of $d$.

Manetti's article that Francesco mentions is a great introduction. Another reference that I am very fond of is Goldman and Millson's paper The Homotopy invariance of Kuranishi space, where they compare the algebraic and analytic description of the hull.

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Thank you for the useful reference! – Piotr Achinger Jan 16 '13 at 0:54