Notes from the talk given at the Quaterly seminar on topology and geometry, Utrecht University, March 11, 2011
After recalling the classical Kodaira-Spencer and Newlander-Nierenberg approaches to deformations of complex manifolds, we will show how these apparently very different descriptions are just two models for the same derived object: the derived global sections of the holomorphic tangent sheaf of the manifold. With this in mind we will see how the periods map of a projective manifold has a very natural and simple local description in terms of oo-groupoids: its classical description is then recovered by globalization and truncation.
The talk will be completely elementary, and no prior knowledge of deformation theory or higher category theory will be assumed.
Given a smooth complex manifold $X$, a deformation of $X$ over a pointed basis $(B,b)$ is a pullback diagram
Here the basis $B$ is a (possibly singular) complex manifold, and $\pi:\mathcal{X}\to B$ is a proper holomorphic function. A morphism of deformations is a morphism of pullback diagrams.
We denote by $Def_X(B)$ the set of isomorphism classes of deformations of $X$ parametrized by the pointed base $(B,b)$
If $\mathcal{X}\to B$ is a deformation of $X$, and $(C,c)\to (B,b)$ is a morphism of pointed complex, then the pullback diagram
exhibits $\mathcal{X}\times_B C\to C$ as a deformation of $X$ over $C$. Therefore $Def_X$ is a functor
We are actually interested in the local behaviour of deformations in a neighborhood of the distinguished point $b$. So $Def_X$ is actually a functor
on germs of pointed complex manifolds.
We pass to infinitesimal deformations by going from germs of pointed complex manifolds to formal pointed manifolds, i.e., to spectra of local $\mathbb{C}$-Artin algebras. Since $Spec$ is a contravariant functor, we finally have the functor of infinitesimal deformations of $X$
where $Art$ is the category of local Artin algebras with residue field $\mathbb{C}$. For an object $A$ in $Art$ we will denote by $\mathfrak{m}_A$ its maxial ideal.
The Newlander-Nierenberg approach to $Def_X$ consists in noticing that from a differential point of view, the family $\mathcal{X}\to B$ is trivial (Ehresmannβs theorem), so we can think of it as a fixed smooth manifold on which we are changing the complex structure. And since the datum of a complex manifold structure on a smooth manifold is the datum of the subsheaf $\mathcal{O}_X$ of holomorphic functions inside the sheaf $C^\infty_X$ of smooth complex valued functions, what we are interested in is in deforming this subsheaf.
We have
where $\mathcal{A}^{p,q}$ is the sheaf of smooth differential forms of type $(p,q)$, and
is the Dolbeault differential. So the idea is: to deform $\mathcal{O}_X$, deform $\overline{\partial}$.
This is achieved as follows: for any $\xi$ in the vector space $A^{0,k}(X,T_X^{1,0})$ of sections of the sheaf of $(0,k)$-forms with coefficients in the holomorphic tangent sheaf? of $X$, consider the holomorphic Lie derivative
where
is the contraction of differential forms with the vector field (with coefficients in $\mathcal{A}^{0,k}_X$) $\xi$.
If $\xi\in A^{0,1}(T_X^{1,0})$, then $\boldsymbol{l}_\xi$ is a degree $(0,1)$ derivation of $\mathcal{A}^{*,*}_X$ and so we can use it to perturb $\overline{\partial}$. Since we want this perturbation to be infinitesimal, we actually fix an Artin algebra $(A,\mathfrak{m}_A)$ and pick $\xi$ in $A^{0,1}(X,T_X)\otimes \mathfrak{m}_A$. Then
is our perturbed Dolbeault operator. By construction it is a derivation of $\mathcal{A}^{*,*}_X$, but it is in general not a differential. We have to impose the condition $(\overline{\partial}+\boldsymbol{l}_\xi)^2=0$, i.e.,
where the bracket is the graded commutator in the graded associative algebra $End^*(A^{*,*}_X)$. The corresponding graded Lie algebra has a natural differential graded Lie algebra structure, with differential given by the adjoint of the Dolbeault operator, so the above equation reads
i.e. is the Maurer-Cartan equation in the dgla $End^*(A^{*,*}_X)$.
Also the graded vector space $A^{0,*}(X,T_X)$ of differential forms of type $(0,*)$ with coefficients in $T_X$ has a natural dgla structure, and the holomorphic Lie derivative
is an morphism of dglas. Therefore the above condition on $\boldsymbol{l}_\xi$ becomes
where $d$ is the differential in the dgla $A^{0,*}(X,T_X)$. Moreover, since $\boldsymbol{l}$ in injective, this is equivalent to
i.e., we can naturally associate a deformation of the complex structure of $X$ to a Maurer-Cartan element in the dgla $A^{0,*}(X,T_X)$. To obtain the set $Def_X$ we still have to quotient out isomorphic copies of the same complex structure. Two Maurer-Cartan elements $\xi_0$ and $\xi_1$ give isomorphic complex structures if there exist a diffeomorphism of $X$ of the form $e^{\boldsymbol{l}_a}$ with $a$ in $A^{0,0}(X;T_X)$ such that the following diagram commutes
In other words
The Newlander-Nierenberg approach to deformations of the sheaf $\mathcal{O}_X$ we described above was based on the remark that $\mathcal{O}_X$ is the kernel of a differenial operator globally defined on $X$. The Kodaira-Spencer approach is instead a local approach. Fix an open cover $\mathcal{U}$ of $X$, so that $\mathcal{O}_U=\mathcal{O}_X\vert_U$ for any $U$ in $\mathcal{U}$. Then the global sheaf $\mathcal{O}_X$ is obatined by glueing toghether the local sheaves $\mathcal{O}_U$ via the trivial glueing maps
The Kodaira-Spencerβs idea is then that to deform $\mathcal{O}_X$ we have to deform the local sheaves $\mathcal{O}_U$ and the glueing maps between the restrictions $\mathcal{O}_U|_{U\cap V}$ and $\mathcal{O}_V\vert_{U\cap V}$. So, one takes an open cover of $X$ such that the βpiecesβ $(U,\mathcal{O}_U)$ have no nontrivial deformations (this happens, e.g., if $(U,\mathcal{O}_U)$ is isomorphic as a ringed space to a polidisk in $\mathbb{C}^n$ with its sheaf of holomorphic functions), then the datum of a deformation of $X$ is entirely encoded in the datum of a deformation of the glueing maps. Since we are working infinitesimally, an isomorphism
will be of the form $\varphi_{UV}=exp(\alpha)$, with $\alpha$ an holomorphic vector field on $U\cap V$. In order to define a global sheaf, the glueings $\varphi_{UV}$ have to satisfy the cocycle condition
Two cocycles $\{\varphi_{UV}\}$ and $\{\psi_{UV}\}$ describe isomorphic deformations if and only if there are isomorphisms $\theta_U:\mathcal{O}_U\to\mathcal{O}_U$ such that the diagram
commutes. And again, since we are working on an infinitesimal base, $\theta_U$ will be of the form $\theta_U=exp(\beta_U)$ with $\beta_U$ an holomorphic vector field on $U$. Therefore we see that deformations of $X$ are given by cohomology classes in nonabelian cohomology
where $\mathcal{T}_X$ is the holomorphic tangent sheaf of $X$. This can be seen as the set of connetced components of the hom-space $\mathbf{H}(X,\mathbf{B}exp(\mathcal{T})$, by looking both at the manifold $X$ and to $\mathbf{B}exp(\mathcal{T})$ as simplicial presheaves over a suitable site of local models for complex geometry. Here $X$ is the presheaf represented by $X$, and $\mathcal{T}$ is the presheaf
Finally, $\mathbf{B}exp(\mathcal{T})$ is the presheaf given by the delooping of
Given an dgla $\mathfrak{g}$, we can functorially associate with any Artin algebra $(A,\mathfrak{m}_A)$ a nilpotent dgla by $(A,\mathfrak{m}_A)\mapsto \mathfrak{g}\otimes \mathfrak{m}_A$. With this in mind we will look at all dglas we will meet in what follows as functors $Art \to nilpotent dgla$.
For $\mathfrak{g}=\bigoplus_{i}\mathfrak{g}^i$ a dgla, let $MC(\mathfrak{g})$ be the set of Maurer-Cartan elements, i.e.,
One thinks element in this set as flat $\mathfrak{g}$-connections: indeed
The subspace $\mathfrak{g}^0$ of $\mathfrak{g}$ is a Lie algebra; the group
acts on as a group of gauge transformations on the set of $\mathfrak{g}$-connections (by conjugation), and this action preserves the subset of flat connections. Hence we have a gauge action of $exp(\mathfrak{g}^0)$ on $MC(\mathfrak{g})$:
Explicitely,
The Deligne groupoid $Del(\mathfrak{g})$ of the dgla $\mathfrak{g}$ is the action groupoid
Note that for $\mathfrak{g}$ a Lie algebra this is the delooping groupoid $\mathbf{B} exp(\mathfrak{g})=*//exp(\mathfrak{g})$.
We denote by $Def(\mathfrak{g}$ the decategorification of $Del(\mathfrak{g})$, i.e., the set of the isomorphism classes of its objects. Also, in what follows we will identify the Deligne groupoid with the simplicial set given by its nerve. For instance, we will write
Consider now two gauge equivalent Maurer-Cartan elements $x_0$ and $x_1$ for a dgla $\mathfrak{g}$. By definition, this means that there exists an element $\gamma$ in $\mathfrak{g}^0$ sucht that $e^\gamma*x_0=x_1$. We can make this way of going from $x_0$ to $x_1$ a βcontinuous pathβ simply by going from the dgla $\mathfrak{g}$ to the dgla $\mathfrak{g}\otimes \Omega_1$, where $\Omega^1$ is the commutative differential graded algebra of differential forms of the geometric realization of the 1-simplex. Indeed, $\gamma\,t$ is a degree zero element in $\mathfrak{g}\otimes \Omega_1$, and $x_0$ is an element in $MC(\mathfrak{g}\otimes\Omega_1)$, and so $x(t)=e^{\gamma\, t}*x_0$ is a Maurer-Cartan element in $\mathfrak{g}\orimes\Omega_1$, with $x(0)=x_0$ and $x(1)=x_1$.
Definition Two Maurer-Cartan elements $x_0$ ad $x_1$ are homotopy equivalent if there exists a maurer-Cartan element $x(t)$ in $mathfrak{g}\otimes\Omega^1$ such that $x(0)=x_0$ and $x(1)=x_1$.
At this level is not even clear that this is indeed an equivalence relation. This is an important point and we will come back to this in a minute.
By the above consideratins it is clear that
Remarkably, also the converse is true (which, by the way shows that homotopy equivalence is an equivalence relation). Indeed, one can show that any Maurer-Cartan element for the dgla $\mathfrak{g}\otimes\Omega_1$ is of the form $x(t)=e^{\gamma(t)}*x_0$, for some $\gamma(t)$ in $\mathfrak{g}^0[t]$ and some $x_0$ in $MC(\mathfrak{g})$. So if $x(t)=e^{\gamma(t)}*x_0$ is a homotopy equivalence between $x_0$ and $x_1$, then $e^{\gamma(1)}$ is a gauge equivalence between them, i.e.
With any nilpotent dgla $\mathfrak{g}$ is therefore naturally associated the simplicial set
where $\Omega_\bullet$ is the simplicial differential graded commutative associative algebra of polynomial (or piecewise smooth, smooth with sitting instnts, etc.) differential forms on the standard $n$-simplexes, for $n\geq0$. The crucial property of the simplicial set $Def_\bullet(\mathfrak{g})$ is that it is a Kan complex, or (in maybe more evocative terms) an oo-groupoid. In particular this gives a much deeper explanation for homotopy equivalence of Maurer-Cartan elements beeing an equivalence relation: this is the horn filling condition for the 2-horn $\Lambda_1[2]$.
If the dgla $\mathfrak{g}$ is concentrated in nonnegative degrees, then parallel transport induces a natural morphism of simplicial sets
Indeed, a Maurer-Cartan element in $\mathfrak{g}\otimes \Omega_n$ is of the form $g+A$ with $g$ a $\mathfrak{g}^1$-valued 0-form on $\Delta[n]$, and $A$ a $\mathfrak{g}^0$-valued 1-form on $\Delta[n]$. The 2-form component of the Maurer-Cartan equation for $g+A$ is $d_\Omega A+\frac{1}{2}[A,A]=0$, where $d_\Omega$ is the differential on differential forms on the simplex. So $A$ is a flat $\mathfrak{g}^0$-connection on the simplex, and parallel transport of $A$ from the vertex 0 of the simplex to the other vertices will define a map from $Def(\mathfrak{g})_n$ to the $n$-simplices of the (nerve of) the Deligne groupoid. Here we are using the fact that the connection $A$ is flat and the $n$-simplex is simply connected.
It is a remarkable result by Hinich that the morphism $\Def_\bullet(\mathfrak{g})\to Del(\mathfrak{g})$ is an acyclic fibration. In particular $\Def_\bullet(\mathfrak{g})$ and $Del(\mathfrak{g})$ are (weakly) homotopy equivalent. An immediate corollary of this is that $\pi_{0}Def_\bullet(\mathfrak{g})=Def(\mathfrak{g})$, but note that by the
mentioned above, we have
for any dgla $\mathfrak{g}$, i.e., even for dglas non concentrated in nonegative degrees. On the other hand, when $\mathfrak{g}$ is not concentrated in nonegative degrees, we do not have, in general, a morphism $\Def_\bullet(\mathfrak{g})\to Del(\mathfrak{g})$ induced by parallel transport. Let us see why. If a degree $-1$ component $\mathfrak{g}^{-1}$ is present, then a Maurer-Cartan element in $\mathfrak{g}\otimes \Omega_n$ is of the form $g+A+h$ with $g$ a $\mathfrak{g}^1$-valued 0-form on $\Delta[n]$, $A$ a $\mathfrak{g}^0$-valued 1-form on $\Delta[n]$, and $h$ a $\mathfrak{g}^{-1}$-valued 2-form on $\Delta[n]$. The 2-form component of the Maurer-Cartan equation for $g+A+h$ is $d_\Omega A+\frac{1}{2}[A,A]+d_{\mathfrak{g}}h+[h,g]=0$, and we see that $A$ is not flat anymore, and that the obstruction to flatness is given by the term $d_{\mathfrak{g}}h+[h,g]$. The parallel transport along the boundary of a 2-simplex then does not gives the identity but an element in the irrelevant stabilizer of the starting vertex, where the irrelevant stabilizer of a Maurer-Cartan element $x$ is defined as
(note that $Stab_{irr}(x)$ is actually a subgroup of the stabilizer $\{ a \in \mathfrak{g}^0\otimes \mathfrak{m}_A \mid e^a*x=x \}$). So we see that the groupoid $\pi_{\leq 1}Def_\bullet(\mathfrak{g})$ fails to be equivalent to $Del(\mathfrak{g})$ precisely because of nontrivial irrelevant stabilizers. And so we also see that we do have a morphism of simlicial sets $Def_\bullet(\mathfrak{g})\to Del(\mathfrak{g})$ as soon as irrelevant stabilizers are trivial, and in this case we have
A particular case of this is the case in which $\mathfrak{g}$ is concentrated in nonnegative degrees we mentioned above.
A very good reason for working with $\infty$-groupoids valued deformation functors rather than with their apparently handier set-valued or groupoid-valued versions is the following folk statement, which allows one to move homotopy constructions back and forth between dglas and (homotopy types of) βniceβ topological spaces.
Theorem The functor $\Def_\bullet: DGLA \to [Art, \infty{-Grpd}]$ is an equivalence of (β,1)-categories.
Here the $(\infty,1)$-category structures involved are the most natural ones, and they are both induced by standard model category structures. Namely, on the model structure on dg-Lie algebras has surjective morphisms as fibrations and quasi-isomorphisms as weak equivalences, just as in the case of differential complexes, whereas the model category structure on the right hand side is the model structure on simplicial presheaves induced by the standard model structure on simplicial sets.
A sketchy proof of the above equivalence can be found in (lurie-moduli); see also (Pridham).
We will refer to a functor $[Art,\infty{-Grpd}]$ as to a formal $\infty$-groupoid. In what follows, we will just say β$\infty$-groupoidβ to mean βformal $\infty$-groupoidβ.
We are now in position to directly relate the Newlander-Nierenberg to the Kodaira-Spencer approach to the infinitesimal deformation theory of a complex manifold $X$. Consider the holomorphic tangent sheaf $\mathcal{T}_X$ of $X$; it is a sheaf of Lie algebras. Then for any open subset $U$ of $X$ we can consider the oo-groupoid $Def_\bullet(\mathcal{T}_X(U))$. This defines a presheaf of oo-groupoids on the site of open subsets of $X$: the oo-presheaf of local deformations of $X$. To pass to global deformations we just have to take (derived) global sections, i.e., the homotopy limit homotopy limit of its local sections over a good open cover $\mathcal{U}$ of $X$. Since $Def_\bullet$ is an equivalence of $(\infty,1)$-categories, we have
On the left-hand side, since $\mathcal{T}$ is a sheaf of Lie algebras, and so $\mathcal{T}_X(U)$ is concentrated in nonnegative degree as a dgla, we have natural homotopy equivalences $Def_\bullet(\mathcal{T}_X(U))\simeq Del(\mathcal{T}_X(U))$, and so
and we find the Kodaira-Spencer description of the deformations of $X$. On the right-hand side, a way of computing the homotopy limit holim $\mathcal{T}_X(\mathcal{U})$, i.e., to compute the derived global sections of the sheaf $\mathcal{T}_X$, is to take a fine resolution? of it, and to take ordinary global sections of this (this is nothing but the general rule that a homotopy limit is the ordinary limit of a fibrant resolution). In particular, by taking as a fine resolution of $\mathcal{T}_X$ the Dolbeault resolution $(\mathcal{A}^{0,*}(T_X),\overline{\partial})$ we find the Newlander-Nierenberg description of $Def_X$.
Since we are now looking at dglas as to an (oo,1)-category, given two dglas $\mathfrak{g}$ and $\mathfrak{h}$, their hom-space $\mathbf{H}(\mathfrak{g},\mathfrak{h})$ is an $\infty$-groupoid. By the equivalence between dglas and (formal) $\infty$-groupoids stated at the end of the previous section, there must be a dgla $\underline{Hom}(\mathfrak {g},\mathfrak{h})$ such that $Def_\bullet(\underline{Hom}(\mathfrak{g},\mathfrak{h})$ is equivalent to $\mathbf{H}(\mathfrak{g},\mathfrak{h})$. And actually the dgla $\underline{Hom}(\mathfrak{g},\mathfrak{h})$ arises in a very natural way and admits a simple explicit description: it is the Chevalley-Eilenberg-type dgla given by the total dgla of the bigraded dgla
endowed with the Lie bracket
defined by
and with the differentials
and
given by
and
These operations are best seen pictorially:
Maurer-Cartan elements in $\underline{Hom}(\mathfrak{g},\mathfrak{h})$ are $L_\infty$-morphisms between $\mathfrak{g}$ and $\mathfrak{h}$. Explicitely, such an $L_\infty$-morphism is a collection of degree $1-n$ maps
such that
Note in particular that a dgla morphism $\varphi:\mathfrak{g}\to\mathfrak{h}$ is, in a natural way, an $L_\infty$-morphism between $\mathfrak{g}$ and $\mathfrak{h}$, of a very special kind: all but the first one of its Taylor coefficients vanish.
Passing to $\pi_{0}$βs, the equivalence $\mathbf{H}(\mathfrak {g},\mathfrak {h})\simeq Def_\bullet(\underline{Hom}(\mathfrak{g},\mathfrak {h}))$ implies the following:
Proposition Let $f,g:\mathfrak{g} \to \mathfrak{h}$ be two $L_\infty$-morphisms of dglas. Then $f$ and $g$ are gauge equivalent in $MC(\underline{\Hom}(\mathfrak {g},\mathfrak{h}))$ if and only if $f$ and $g$ represent the same morphism in the homotopy category of dglas.
Let now $\mathfrak{g}$ and $\mathfrak{h}$ be dglas and $\boldsymbol{i}: \mathfrak{g}\to \mathfrak{h}[-1]$ be a morphism of graded vector spaces. Then $\boldsymbol{i}$, and so also $-\boldsymbol{i}$, is an element of $\underline{Hom}^{-1,1}(\mathfrak{g},\mathfrak{h})$, and so a degree zero element in the dgla $\underline{Hom}(\mathfrak{g},\mathfrak{h})$. The gauge transformation $e^{-\boldsymbol{i}}$ will map the $0$ dgla morphism to an $L_\infty$-morphism $e^{-\boldsymbol{i}}*0$ between $\mathfrak{g}$ and $\mathfrak{h}$.
This $L_\infty$-morphism will in general fail to be a dgla morphism since its nonlinear components will be nontrivial. This is conveniently seen as follows: let $\boldsymbol{l}=d_{1,0}\boldsymbol{i}$; that is,
for any $a\in\mathfrak{g}$. Then the $(0,1)$-component of
is just $\boldsymbol{l}$; the $(-1,2)$-component is
and, for $n\geq 3$ the $(1-n,n)$-component has two contributions, one of the form
and the other of the form
From this we see that all the nonlinear components of $e^{-\boldsymbol{i}}*0$ vanish as soon as one imposes the two simple conditions
These two condition may appear quite unnatural, but are actually implied by the following two, which are stronger, simpler, and familiar from differential geometry:
Definition A linear map $\boldsymbol{i}\colon \mathfrak{g}\to \mathfrak{h}[-1]$ satisfying the two conditions above will be called a Cartan homotopy.
The name Cartan homotopy has an evident geometric origin: if $\mathcal{T}_X$ is the tangent sheaf of a smooth manifold $X$ and $\Omega^*_{X}$ is the sheaf of complexes of differential forms, then the contraction of differential forms with vector fields is a Cartan homotopy
In this case, $\boldsymbol{l}_a$ is the Lie derivative along the vector field $a$, and the conditions $\boldsymbol{i}_{[a,b]}= [\boldsymbol{i}_a, \boldsymbol{l}_b]$ and $[\boldsymbol{i}_a, \boldsymbol{i}_b]=0$, together with the defining equation $\boldsymbol{l}_a=[d_{\Omega^*_X},\boldsymbol{i}_a]$ and with the equations $\boldsymbol{l}_{[a,b]}=[\boldsymbol{l}_a,\boldsymbol{l}_b]$ and $[d_{\Omega^*_X},\boldsymbol{l}_a]=0$ expressing the fact that
is a dgla morphism, are nothing but the well-known Cartan identities involving contractions and Lie derivatives.
The above discussion can be summarized as follows.
Proposition Let $\mathfrak{g}$ and $\mathfrak{h}$ be two dglas. If $\boldsymbol{i}\colon \mathfrak{g}\to\mathfrak{h}[-1]$ is a Cartan homotopy, then $\boldsymbol{l}=d_{1,0}\boldsymbol{i}\colon \mathfrak {g}\to \mathfrak{h}$ is a dgla morphism gauge equivalent to the zero morphism via the gauge action of $e^{\boldsymbol{i}}$. In other wors, the pair $(\boldsymbol{l},e^{\boldsymbol{i}})$ defines a morphism form $\mathfrak{g}$ to the homotopy fiber of $\mathfrak{h}\stackrel{Id}{\to}\mathfrak{h}$, what is known as the inner derivations dgla $inn(\mathfrak{h})$.
By the above considerations we immediately see that, if the morphism $\boldsymbol{l}:\mathfrak{g}\to \mathfrak{h}$ factors through a dgla morphism $f:\mathfrak{n}\to \mathfrak{h}$, then we have a homotopy commutative diagram
and so, by the universal property of the homotopy pullback, a dgla morphism
from $\mathfrak{g}$ to the homotopy fiber of $f:\mathfrak{n}\to \mathfrak{h}$.
Taking $Def_\bullet$βs we therefore obtain a morphism of $\infty$-groupoids:
which, taking $\pi_0$βs gives a natural morphism of sets
from the classical deformation fuctor associated with the dgla $\mathfrak{g}$ to the set of connected components of the homotopy fiber of $Def_\bullet(\mathfrak{n})\to Def_\bullet(\mathfrak{h})$.
To investigate the geometry of $\pi_{0}(Def_\bullet(\mathfrak{n})\times_{Def_\bullet(\mathfrak{h})}^{f}*)$ it is sufficiet to recall how it fits into the homotopy exact sequence
inducing a canonical isomorphism
The group $\pi_1(Def_\bullet(\mathfrak{h})$ is the group of automorphisms of the distinguished object $0$ in the groupoid $\pi_{\leq 1}\Def_\bullet(\mathfrak{h}))$; as we have remarked, this groupoid is not equivalent to the Deligne groupoid of $\mathfrak{h}$, since the irrelevant stabilizer of a Maurer-Cartan element $x$ may be nontrivial. However, the group $\pi_1(Def_\bullet(\mathfrak{h})$ only sees the connected component of the distinguished element $0$, and on this connected component the irrelevant stabilizers are trivial as soon as the differential of the dgla $\mathfrak{h}$ vanishes on $\mathfrak{h}^{-1}$. This immediately follows from noticing that
an by the fact that irrelevant stabilizers of gauge equivalent Maurer-Cartan elements are conjugate subgroups of $\exp(\mathfrak{h}^0)$. In particular, if $\mathfrak{h}$ is a graded Lie algebra (which we can consider as a dgla with trivial differential), then $\pi_1(\Def(\mathfrak{h});0)\simeq \exp(\mathfrak{h}^0)$. Then, if $j:\mathfrak {n}\hookrightarrow\mathfrak{h}$ is a subdgla of $\mathfrak{h}$, we also have $\pi_1\Def(\mathfrak{n})\cong \exp(\mathfrak{n}^0)$, and the map $Def(\mathfrak{g})\to\pi_{0}(Def_\bullet(\mathfrak{n})\times_{Def_\bullet(\mathfrak{h})}^{j}*)$ is just the natural map
which sends a Maurer-Cartan element $\xi\in \mathfrak{g}^1$ to $e^{\boldsymbol{i}_\xi}\mod \exp(\mathfrak {n}^0)$.
A particularly interesting case of this situation is when the morphism $f:\mathfrak{n}\to\mathfrak{h}$ is formal, i.e., when it is homotopy equivalent to an inclusion in cohomology $H^*(\mathfrak {n})\hookrightarrow H^*(\mathfrak{h})$. Indeed, in this case the morphism $f:Def_\bullet(\mathfrak{n})\to Def_\bullet(\mathfrak{h})$ will be homotopy equivalent to the morphism $j:Def_\bullet(H^*(\mathfrak{n}))\to Def_\bullet(H^*(\mathfrak{h}))$ and there will be an induced isomorphism between $\pi_1(Def(\mathfrak{ h})/f_*\pi_1\Def(\mathfrak{n})$ and the homogeneous space $exp(H^0(\mathfrak{h}))/exp(H^0(\mathfrak{n}))$. We can summarize the results described in this section as follows:
Proposition Let $\boldsymbol{i}\colon \mathfrak{g}\to\mathfrak{h}[-1]$ be a Cartan homotopy, let $\boldsymbol{l}\colon \mathfrak{g}\to\mathfrak{h}$ be the associated dgla morphism, and let $f:\mathfrak{n}\hookrightarrow\mathfrak{h}$ be a dgla morphism factorizing $\boldsymbol{l}$. Then, if the morphism $f$ is formal, we have a morphism
induced by the dgla map $\mathfrak{g} \to \mathfrak{n}\times_{\mathfrak{h}}^{f}\{0\}$.
It should be remarked that the morphism $\mathcal{P}$ is not canonical: it depends on the choice of a quasi isomorphism beween $f:\mathfrak {n}\to\mathfrak{h}$ and $j:H^*(\mathfrak{n})\hookrightarrow H^*(\mathfrak{n}))$.
The differential of $\mathcal{P}$ is easily computed: it is the linear map
Since the model category structure on dglas is the same as on differential complexes, we can compute the $H^1$ on the right hand side by computing the homotopy fiber of $f:\mathfrak{n}\to \mathfrak{h}$ in the category of complexes. In particular, if $j:\mathfrak{n}\hookrightarrow\mathfrak{h}$ is an inclusion, then the natural quasi-isomorphism of complexes
tells us that the differential of $\mathcal{P}$ is just the map
induced by the morphism of complexes ${\boldsymbol{i}}\colon \mathfrak{g} \to (\mathfrak{h}/\mathfrak{n})[-1]$.
Also note that if the inclusion $j:\mathfrak{n}\hookrightarrow\mathfrak{h}$ is formal, then we have a (non canonical) isomorphism
$H^0(\mathfrak{h})/H^0(\mathfrak{n})$ and $H^0(\mathfrak{h}/\mathfrak{n})$, in agreement with the descripion of $\mathcal{P}$ given in the above proposition.
Let again $X$ be a smooth projective manifold, and let $\mathcal{T}_X$ and $\Omega_X^*$ be the holomorphic tangent sheaf and the sheaf of holomorphic differential forms on $X$, respectively. The sheaf of complexes $(\Omega_X^*,d_{\Omega_X^*})$ is naturally filtered by setting $F^p\Omega_X^*=\oplus_{i\geq p}\Omega^i_X$. Finally, let $\mathcal{E} n d^*(\Omega_X^*)$ be the endomorphism sheaf of $\Omega_X^*$ and $\mathcal{E} n d^{\geq 0}(\Omega_X^*)$ be the subsheaf consisting of nonnegative degree elements. Note that $\mathcal{E} n d^{\geq 0}(\Omega_X^*)$ is a subdgla of $\mathcal{E} n d^{*}(\Omega_X^*)$, and can be seen as the subdgla of endomorphisms preserving the filtration on $\Omega_X^*$.
Recall that the prototypical example of Cartan homotopy was the contraction of differential forms with vector fields ${\boldsymbol{i}}: \mathcal{T}_X \to \mathcal{E} n d^*(\Omega_X^*)[-1]$; the corresponding dgla morphism is $a\mapsto \boldsymbol{l}_a$, where $\boldsymbol{l}_a$ the Lie derivative along $a$. Explicitly, $\boldsymbol{l}_a= d_{\Omega_X^*}\circ {\boldsymbol{i}}_a+{\boldsymbol{i}}_{a}\circ d_{\Omega_X^*}$, and so $\boldsymbol{l}_a$ preserves the filtration and $\boldsymbol{l}:\mathcal{T}_X\to\mathcal{E} n d^*(\Omega_X^*)$ factors through the inclusion $j:\mathcal{E} n d^{\geq 0}(\Omega_X^*)\hookrightarrow \mathcal{E} n d^*(\Omega_X^*)$.
Therefore, we have a natural morphism of oo-groupoids
The homotopy fiber on the right should be thought as a homotopy flag manifold. Let us briefly explain this: the functor $Def_\bullet(\mathcal{E} n d^{*}(\Omega_X^*))$ describes the infinitesimal deformations of the differential complex $\Omega_X^*$, whereas the functor $Def_\bullet(\mathcal{E} n d^{\geq0}(\Omega_X^*))$ describes the deformations of the filtered complex $(\Omega_X^*,F^\bullet\Omega_X^*)$, i.e., of the pair consisting of the complex $\Omega_X^*$ and of the filtration $F^\bullet\Omega_X^*$. Therefore, the homotopy fiber describes a deformation of the pair (complex, filtration) together with a trivialization of the deformation of the complex.
Summing up, the contraction of differential forms with vector fields induces a map of presheaves of oo-groupoids (on the category of open subsets of $X$)
which we will call the local periods map of $X$.
To recover from this the classical periods map, we just need to take global sections (clearly, since we are working in homotopy categories, these will be derived global sections). Globalizing the Cartan homotopy ${\boldsymbol{i}}: \mathcal{T}_X \to \mathcal{E} n d^*(\Omega_X^*)[-1]$, we get a Cartan homotopy
The image of the corresponding dgla morphism $\boldsymbol{l}$ (the derived globalization of the Lie derivative) preserves the filtration $F^\bullet\mathbf{R}\Gamma\Omega_X^*$ induced by $F^\bullet\Omega_X^*$, so we have a natural map of $\infty$-groupoids
and, at the $\pi_0$ level, a map of sets
On the left hand side we see $Def_X$, the Set-valued functor of (classical) infinitesimal deformations of $X$. On the right hand side we are considering the homotopy fiber of the inclusion $End^*(\mathbf{R}\Gamma\Omega_X^*;F^\bullet\mathbf{R}\Gamma\Omega_X^*)\hookrightarrow End^*(\mathbf{R}\Gamma\Omega_X^*)$ of the subdgla consisting of endomorhisms preserving the filtration. And we know from Hodge theory that this inclusion is formal, and that
where $F^\bullet H^*(X;\mathbb{C})$ is the Hodge filtration on the cohomology of $X$. Moreover,
and so, by results described in the previous section,
Thus we recover the classical periods map of $X$
Also, the general argument above tells us that the differential of $\mathcal {P}$ is the map induced in cohomology by the contraction of differential forms with vector fields,
a result originally proved by Griffiths [griffiths]. In the above formula, $\int_p$ denotes the end of the diagram
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