The rational homotopy type of a topological space X is the commutative differential graded algebra (cdga for short) in the homotopy category defined by inverting quasi-isomorphisms, where is Sullivan’s functor of rational piece-wise linear forms. A topological space X is said to be formal if its rational algebra of piece-wise linear forms is a formal cdga: there is a string of quasi-isomorphisms from to its cohomology considered as a cdga with trivial differential. In particular, if X is formal then its rational homotopy type is completely determined by its cohomology ring, and higher order Massey products vanish. Using Hodge theory, Deligne, Griffiths, Morgan and Sullivan  proved that smooth projective varieties or, more generally, compact Kähler manifolds, are formal.
Simpson  showed that every finitely presented group G is the fundamental group of an irreducible projective variety X. Later, Kapovich and Kollár  showed that X can be chosen to be a complex projective surface with simple normal crossing singularities only. This implies the existence of non-formal complex projective varieties. For instance, one may take G to be the fundamental group of the complement of the Borromean rings, which has non-trivial triple Massey products. In this paper, we use mixed Hodge theory to show that a large class of projective varieties with normal isolated singularities are formal topological spaces. We also study the mixed Hodge structures on the rational homotopy type of a contraction of a subvariety and prove analogous results in this setting.
We next outline the contents of this paper. For a complex algebraic variety X, the complex homotopy type can be computed from the differential bigraded algebra defined by the first term of the multiplicative weight spectral sequence, a multiplicative analogue of Deligne’s weight spectral sequence . This result was proven by Morgan  for smooth quasi-projective varieties and by Cirici–Guillén  in the possibly singular case. In Section 2, we study the multiplicative weight spectral sequence of a projective variety with isolated singularities and provide a simple description of its algebra structure in terms of a resolution of singularities of the variety. This gives an upper bound on the homotopy theoretic complexity of the variety.
The idea that purity of the weight filtration implies formality is part of the folklore on mixed Hodge theory and goes back to . In Section 3, we give a simple proof of a refinement of this idea: we show that the purity of the weight filtration of a complex projective variety up to a certain degree, implies formality of the variety up to the same degree. We prove the main results of this paper in Section 4. We first show, using the multiplicative weight spectral sequence, that every normal complex projective surface is formal. We then generalize this result to arbitrary dimensions and prove formality for normal projective varieties with isolated singularities whose link is -connected, where n is the dimension of the variety. In particular, complete intersections with isolated singularities are formal. Using similar techniques, we show that if X is a projective variety with normal isolated singularities admitting a resolution of singularities with smooth exceptional divisor, then X is a formal topological space. Lastly, in Section 5 we prove analogous results for contractions of subvarieties. In particular, we show that if is a closed immersion of smooth projective varieties, then is a formal topological space.
The results of this paper arose from a more general study (see ) of the vanishing of Massey products on the intersection cohomology of projective varieties with only isolated singularities. However, the simplicity of the statements and proofs in the classical setting of rational homotopy encouraged us to write them up separately. Our more general study follows the intersection-homotopy treatment of , where Chataur, Saralegi and Tanré prove intersection-formality for certain spaces. In particular, they show that any nodal hypersurface in is formal.
2 Multiplicative weight spectral sequence
Deligne proved that the k-th cohomology space of every complex algebraic variety X carries a functorial mixed Hodge structure: this is given by an increasing filtration
of the rational cohomology of X, called the weight filtration, together with a decreasing filtration F of the complex cohomology , called the Hodge filtration, such that F and its complex conjugate induce a Hodge decomposition of weight p on each graded piece . If X is smooth then , while if X is projective then . Let us briefly explain how W is defined in the latter case. Let be a simplicial resolution of X: this is a smooth simplicial variety together with an augmentation morphism satisfying cohomological descent (see , see also [24, Section II.5]). Deligne showed that the associated spectral sequence degenerates at the second stage, and that the induced filtration on the rational cohomology of X is well-defined (does not depend on the chosen resolution) and is functorial for morphisms of varieties. The weight filtration W on is then defined by décalage of the induced filtration. We have
Let . The weight filtration on is said to be pure of weight k if
For instance, if X is smooth and projective then W on is pure of weight k, for all . A well-known consequence of the Decomposition Theorem of intersection homology is that a projective variety X with only isolated singularities satisfies semi-purity: the weight filtration on is pure of weight k, for all , where (see , see also  for a more direct proof).
Morgan  introduced mixed Hodge diagrams of differential graded algebras and proved the existence of functorial mixed Hodge structures on the rational homotopy groups of smooth complex algebraic varieties. His results were extended to the singular setting by Hain  and Navarro-Aznar  independently. In particular, Navarro-Aznar introduced the Thom–Whitney simple functor and developed the construction of algebras of piece-wise linear forms associated to simplicial varieties. As a result, for any complex algebraic variety X, one has a multiplicative weight spectral sequence which is a well-defined algebraic invariant of X in the homotopy category of differential bigraded algebras, and is homotopy equivalent, when forgetting the algebra structure, to Deligne’s weight spectral sequence . Furthermore, the multiplicative weight spectral sequence carries information on the rational homotopy type of the variety.
2.1 Mixed Hodge diagrams and multiplicative weight spectral sequence
We next recall the definition of the multiplicative weight spectral sequence associated with a complex algebraic variety.
A filtered cdga over a field is a cdga A over together with an (increasing) filtration indexed by such that , , and .
Every filtered cdga has an associated spectral sequence, each of whose stages is a differential bigraded algebra with differential of bidegree .
A mixed Hodge diagram (of cdgas over ) consists of a filtered cdga over , a bifiltered cdga over , together with a string of filtered quasi-isomorphisms from to . In addition, the following axioms are satisfied:
The weight filtration W is regular and exhaustive. The Hodge filtration F is biregular. The cohomology has finite type.
For all , the differential of is strictly compatible with F.
For all and all , the filtration F induced on defines a pure Hodge structure of weight on .
Such a diagram is denoted as
By forgetting the multiplicative structures we recover the original notion of mixed Hodge complex introduced by Deligne (see [8, Section 8.1]). In particular, the k-th cohomology group of every mixed Hodge diagram is a mixed Hodge structure.
The following is a multiplicative version of Deligne’s Theorem  on the existence of functorial mixed Hodge structures in cohomology.
For every complex algebraic variety X there exists a mixed Hodge diagram such that and for all , the cohomology is isomorphic to Deligne’s mixed Hodge structure on . This construction is well-defined and functorial for morphisms of varieties in the homotopy category of mixed Hodge diagrams.
The rational component of every mixed Hodge diagram is a filtered cdga over the rationals. Hence it has an associated spectral sequence .
([6, Theorem 3.23])
Let be mixed Hodge diagram such that . There is a string of quasi-isomorphisms of complex cdgas from to compatible with the filtration W.
The proof of the above result uses minimal models in the sense of rational homotopy. This is why we ask that mixed Hodge diagrams are cohomologically connected. Under some finite type conditions the above result is also valid over (see [6, Theorem 2.26]). We remark that in , a weaker notion of mixed Hodge diagram is used. This does not affect the above result.
Let X be a complex algebraic variety. The multiplicative weight spectral sequence of X is the spectral sequence
associated with the filtered cdga given by the rational component of .
This is a well-defined algebraic invariant of X in the homotopy category of differential bigraded algebras. When forgetting the multiplicative structures, there is a homotopy equivalence of bigraded complexes between and Deligne’s weight spectral sequence .
The main advantage of the multiplicative weight spectral sequence with respect to Deligne’s weight spectral sequence is that by Theorem 2.4, the former carries information about the rational homotopy type of X.
2.2 Thom–Whitney simple
Theorem 2.3 relies on the Thom–Whitney simple functor, which associates, to every strict cosimplicial cdga over a field of characteristic zero, a new cdga over . This cdga is homotopically equivalent, as a complex, to the total complex of the original cosimplicial cdga. Hence the Thom–Whitney simple can be viewed as a multiplicative version of the total complex. We next recall its construction. For every , denote by the cdga given by
where denotes the free cdga over generated by in degree 0 and in degree 1. The differential on is defined by and . For all , define face maps by letting
These definitions make into a strict simplicial cdga (the adjective strict accounts for the fact that we do not require degeneracy maps).
Recall that the total complex of a strict cosimplicial cochain complex is given by
where denotes the cochain complex of . Analogously, we have:
Let be a strict cosimplicial cdga over . The Thom–Whitney simple of is the cdga over defined by the end
Let be morphisms of cdgas. Then is given by the pull-back
where is the evaluation map given by and .
We shall need the following filtered version of the Thom–Whitney simple. Given , consider on the cdga the multiplicative increasing filtration defined by letting be of weight 0 and be of weight , for all . Note that is the trivial filtration, while is the bête filtration.
Let be a strict cosimplicial filtered cdga. The r-Thom–Whitney simple of is the filtered cdga defined by
To study the weight spectral sequence we shall mostly be interested in the behavior of the filtration of the Thom–Whitney simple, which in the setting of complexes corresponds to the diagonal filtration of introduced in [8, Section 7.1.6] (see also [7, Section 2]). The following is a matter of verification.
Let be a finite strict cosimplicial filtered cdga. The spectral sequence associated with satisfies
Deligne’s simple of a cosimplicial mixed Hodge complex is the diagram of complexes given by
which by [8, Theorem 8.1.15], is again a mixed Hodge complex. The following Lemma is a multiplicative analogue of this result and gives a Thom–Whitney simple in the category of mixed Hodge diagrams.
Let be a strict cosimplicial mixed Hodge diagram. The diagram of cdgas given by
is a mixed Hodge diagram, which is homotopy equivalent, as a complex, to Deligne’s simple .
2.3 Main examples
We next give a description of the multiplicative weight spectral sequence in some particular situations of interest for this paper.
Smooth projective varieties
Let X be a smooth projective variety. A mixed Hodge diagram for X is given by the data and the complex de Rham algebra, with W the trivial filtration and F the classical Hodge filtration. The multiplicative weight spectral sequence satisfies and for all .
Varieties with normal crossings
Let be a simple normal crossings divisor in a smooth projective variety of dimension n. We may write as the union of irreducible smooth varieties meeting transversally. Let and for all , denote by the disjoint union of all p-fold intersections where denotes an ordered subset of . Since D has normal crossings, it follows that is a smooth projective variety of dimension . For , denote by the inclusion and let . This defines a simplicial resolution , called the canonical hyperresolution of D. Deligne’s weight spectral sequence is the first quadrant spectral sequence given by (see for example [14, §4])
with the differential defined via the combinatorial restriction morphisms
By computing the cohomology of we obtain
Let be the canonical hyperresolution of a simple normal crossings divisor D. The multiplicative weight spectral sequence of D is given by
Since is smooth and projective for each p, there is a strict cosimplicial mixed Hodge diagram with trivial weight filtrations. A mixed Hodge diagram for D is then given by the Thom–Whitney simple . Indeed, cohomological descent for rational homotopy gives a quasi-isomorphism . Furthermore, the filtrations of given in Lemma 2.10 make into a mixed Hodge diagram. By Lemma 2.10, when forgetting the multiplicative structures, is quasi-isomorphic to the canonical mixed Hodge complex for D (see also [14, §4]). Hence it induces Deligne’s mixed Hodge structure on the cohomology of D. The result follows from Lemma 2.9, by noting that and for . ∎
Let D be a simple normal crossings divisor of complex dimension 1. Then is a disjoint union of smooth projective curves, is a collection of points and for all . Deligne’s weight spectral sequence is given by
where . The multiplicative weight spectral sequence is given by
where the term is given by the pull-back
and the differential is given by . The products and are given by
respectively. The unit is . The maps and given by
define an inclusion of bigraded complexes which is a quasi-isomorphism.
Smooth quasi-projective varieties
Let X be a smooth projective variety and let be a smooth compactification of X such that the complement is a union of divisors with normal crossings. A mixed Hodge diagram for X is defined via the algebra of forms on which have logarithmic poles along D (see [21, §3] for details, see also [23, §8]). In this case, the multiplicative weight spectral sequence for X coincides with Deligne’s weight spectral sequence, given by
The differential is given by the combinatorial Gysin map
where are the inclusion maps. The algebra structure of is induced by the combinatorial restriction morphisms together with the cup product of for .
Let X be a complex projective variety of dimension n with only isolated singularities and denote by Σ the singular locus of X. By Hironaka’s Theorem on resolution of singularities there exists a cartesian diagram
where is smooth, is a proper birational morphism which is an isomorphism outside Σ and is a simple normal crossings divisor. Since both and Σ are smooth and projective, there are mixed Hodge diagrams and for and Σ respectively with trivial weight filtration. Let be a mixed Hodge diagram for D as constructed in the proof of Proposition 2.11. The maps and defined by composing the map with j and g respectively induce morphisms of mixed Hodge diagrams and .
Let X be a complex projective variety with only isolated singularities. With the above notation we have:
A mixed Hodge diagram for X is given by
For , the term is given by the pull-back
while for and we have
The differential of is defined component-wise, via the differential of and the differentiation with respect to t.
Denote by the combinatorial restriction maps, with . Let
be given by . Then
By Lemma 2.10 the Thom–Whitney simple of a strict cosimplicial mixed Hodge diagram, with the filtrations and , is a mixed Hodge diagram. Hence is a mixed Hodge diagram, which by cohomological descent satisfies . By forgetting the multiplicative structures, we obtain a mixed Hodge complex for X (see for example [11, Section 2.9]). This proves (1). Assertion (2) now follows from (1) and Lemma 2.9. Assertion (3) is a matter of verification. ∎
3 Purity implies formality
In this section we give a simple proof of the fact that the purity of the weight filtration of a complex projective variety up to a certain degree, implies formality of the variety up to the same degree. A direct application is the formality of the Malcev completion of the fundamental group of projective varieties with normal isolated singularities.
Let be an integer. A morphism of cdgas is called r-quasi-isomorphism if the induced morphism in cohomology is an isomorphism for all and a monomorphism for .
A cdga over is called r-formal if there is a string of r-quasi-isomorphisms from to its cohomology considered as a cdga with trivial differential. We will say that a topological space X is r-formal if the rational cdga is r-formal.
The case is of special interest, since 1-formality implies that the rational Malcev completion of can be computed directly from the cohomology group , together with the cup product
In this case we say that is formal. For we recover the usual notion of formality, which in the case of simply connected (or more generally, nilpotent) spaces, implies that the higher rational homotopy groups , with can be computed directly from the cohomology ring . Note that if X is formal, then is also formal.
Let X be a complex projective variety and let be an integer. If the weight filtration on is pure of weight k, for all , then X is r-formal.
We prove formality over and apply independence of formality on the base field for cdgas with cohomology of finite type (see , see also ). Since the disjoint union of r-formal spaces is r-formal, we may assume that X is connected, so it has a mixed Hodge diagram with . By Theorem 2.4 it suffices to define a string of r-quasi-isomorphisms of differential bigraded algebras from to . Let M be the bigraded vector space given by for all and for all and all , where denotes the differential of . Then is a bigraded sub-complex of with trivial differential. Denote by the inclusion. Since , the multiplicative structure induced by φ on M is closed in M. Hence φ is an inclusion of differential bigraded algebras. On the other hand, we have
This gives an inclusion of bigraded algebras . Assume that the weight filtration on is pure of weight k, for all . We next show that both φ and ψ are r-quasi-isomorphisms. Indeed, for every and every such that we have , while for every , we have . Therefore the induced maps and are isomorphisms for all and the maps and are monomorphisms. ∎
We highlight the two extreme cases and in the following corollary.
Let X be a complex projective variety.
If the weight filtration on is pure of weight 1 , then is formal.
If the weight filtration on is pure of weight k , for all then X is formal.
Let X be a complex projective variety of dimension n. Assume that X is a -homology manifold (for all , for and ). Then the weight filtration on is pure of weight k, for all (see [8, Theorem 8.2.4]). Hence X is formal. Examples of such varieties are given by weighted projective spaces or more generally V-manifolds (see [10, Appendix B]), surfaces with -singularities, the Cayley cubic or the Kummer surface.
In fact, purity of the weight filtration is strongly related to Poincaré duality: if X is a complex projective variety whose rational cohomology satisfies Poincaré duality, then the weight filtration on is pure of weight k, for all . Indeed, the Poincaré duality maps are compatible with mixed Hodge structures. The weights on the left-hand (resp. right-hand) side are (resp. ), hence equal to k. Therefore such varieties are formal (cf. [16, Theorem 5]). Another well-known result relating purity and Poincaré duality is the purity of the weight filtration on the (middle perversity) intersection cohomology of a projective variety X. Furthermore, Weber  showed that for a complex projective variety X, the image of the map is isomorphic to the pure term .
The purity of the weight filtration in cohomology does not imply Poincaré duality, as shown by the following example.
(Projective cone over a smooth curve)
Let be a smooth curve of genus g and consider the projective cone over C. The Betti numbers of X are , , , and . The weight filtration on is pure of weight k, for all , and hence X is formal, but does not satisfy Poincaré duality.
In , it is shown that the weight filtration on of a normal complex projective variety X is pure of weight 1. In the case of isolated singularities, the proof is a standard argument in mixed Hodge theory.
Let X be a normal complex projective variety with isolated singularities. Then the weight filtration on is pure weight 1.
Let Σ denote the singular locus of X and a resolution such that is a simple normal crossings divisor. Since X is normal, by Zariski’s main Theorem we have . Since , we have for all . This gives a Mayer–Vietoris long exact sequence
which is strictly compatible with the weight filtration (see for example [24, Corollary-Definition 5.37]). Since is smooth and projective, its weight filtration is pure. Hence the weight filtration on is pure weight 1. ∎
The fundamental group of every normal complex projective variety X with isolated singularities is formal.
4 Formality of projective varieties with isolated singularities
By purely topological reasons we know that every simply connected, 4-dimensional CW-complex is formal. We also know there exist non-formal 4-dimensional CW-complexes. As mentioned in the introduction, thanks to deep results of Simpson and Kapovich–Kollár we know that there exist non-formal complex projective surfaces. In this section we prove that every complex projective surface with normal singularities is formal. We generalize this result in two directions. First, we prove formality for projective varieties of dimension n with only isolated singularities whose link is -connected. Second, we prove formality for those projective varieties with normal isolated singularities admitting a resolution of singularities with smooth exceptional divisor.
Every normal complex projective surface is a formal topological space.
Let X be a normal complex projective surface, which we may assume to be connected. We use the formulas for given in Proposition 2.13. Since normal singularities have codimension , we have . Since , we have for all . By Lemma 3.7 together with semi-purity, the weight filtration on is pure of weight k for all . We have
For all take a section of the projection . This defines a morphism of bigraded complexes which is a quasi-isomorphism. We next show that ρ is multiplicative. By bidegree reasons, the only non-trivial products in are the products
induced by the cup product of . Since ρ is the identity on , it preserves these products. It also preserves the unit . It only remains to see that the diagram
commutes. By Proposition 2.13, the term is given by the pull-back
and for . Moreover, the differential is given by and the product is given by .
Let . Since , it follows that if and only if . Therefore we have , and the above diagram commutes. This proves that the map
is multiplicative. Since X is connected, it has a mixed Hodge diagram with . Hence by Theorem 2.4 we have a string of quasi-isomorphisms of complex cdgas
To conclude that X is formal it suffices to apply descent of formality of cdgas from to . ∎
Let C be a curve of degree with nodes in . The genus of C is given by . Choose a smooth projective curve of degree intersecting C transversally at smooth points of C, so that and consider the blow-up of at the points of . Then the proper transform of C has negative self-intersection and we may consider the blow-down X of to a point. Explicitly, assume that the curve C is given by , and that is given by . Then X is the projective variety defined by the equation which has a normal isolated singularity at . Here are the homogeneous coordinates in . The normalization of is a smooth projective curve of genus g and is homeomorphic to the connected sum of projective planes. Deligne’s weight spectral sequence can be written as
Hence has a non-trivial weight filtration:
Since X is simply connected (see for example [10, Corollary V.2.4]), we may compute the rational homotopy groups of X with their weight filtration from a bigraded minimal model of the bigraded algebra . The weight filtration on satisfies , where the term denotes the indecomposables of M of bidegree .
The cohomology ring of is given by with , and for all . Here T denotes the top class of , a is the hyperplane class and correspond to the exceptional divisors. Let . Then with and for . Hence we may write
where the generators have bidegree , and . By bidegree reasons, the only non-trivial products are given by and , for all . We compute the first steps of a minimal model for . Let be the free bigraded algebra
with trivial differential generated by elements of bidegree , and . Then the map given by is a 2-quasi-isomorphism of bigraded algebras. Hence we have
Let where are the graded vector spaces of pure bidegree , and are the differentials given by
Then the extension of given by is a 3-quasi-isomorphism. The formula gives
For example, we may take C to be the nodal cubic curve given by and a smooth plane quartic. Then , and . This gives
The following is a generalization of Theorem 4.1 to projective varieties of arbitrary dimension.
Let X be a complex projective variety of dimension n with normal isolated singularities. Denote by Σ the singular locus of X, and for each let denote the link of σ in X. If for all for every , then X is a formal topological space.
The link of in X is a smooth connected real manifold of dimension . Let . Then . Assume that for all . By Poincaré duality the only non-trivial rational cohomology groups of L are in degrees and . Let . From the Mayer–Vietoris exact sequence
it follows that the map is an isomorphism whenever or , and injective for . Since has weights in and has weights in , and the morphism is strictly compatible with the weight filtrations, it follows that for , the weight filtration on is pure of weight k. Furthermore, by semi-purity we have that is pure of weight . Therefore the only non-trivial weights of are in degree . The weight spectral sequence for X has the form
where the bullets denote the non-trivial elements. Consider the quasi-isomorphism of complexes
defined by taking sections of the projections . We next show that ρ is multiplicative. Note that by bidegree reasons, the only non-trivial products of are between elements of the first column. Since ρ is the identity on , it preserves these products. It also preserves the unit . Since for all , we have for all . Therefore it only remains to show that for , the following diagram commutes:
By Proposition 2.13 and since for , the term is given by
while for we have . The proof now follows as in the proof of Theorem 4.1. ∎
Let X be a complete intersection of dimension . Assume that the singular locus is a finite number of points. The link of in X is -connected (this result is due to Milnor  in the case of hypersurfaces and to Hamm  for general complete intersections). Therefore by Theorem 4.3, X is formal. Note that in particular, every complex hypersurface with isolated singularities is formal.
Let X be a projective variety with only isolated singularities. Assume that there exists a resolution of singularities such that the exceptional divisor is smooth. Then X is a formal topological space.
We may assume that X is connected. By Proposition 2.13 the multiplicative weight spectral sequence is given by
The differential is given by . The non-trivial products of are the maps given by and the maps given by . The unit is . By computing the cohomology of we find
with the non-trivial products being . Now define a quasi-isomorphism of bigraded complexes as follows: Let be defined by the inclusion, for all . Define by taking a section of the projection , for all .
To see that ρ is a morphism of bigraded algebras it suffices to observe that it preserves the unit and that . Since X is connected, it has a mixed Hodge diagram with . Hence by Theorem 2.4 we have a string of quasi-isomorphisms of complex cdgas
To conclude that X is formal it suffices to apply descent of formality of cdgas from to . ∎
Projective varieties with only isolated ordinary multiple points are formal. Projective cones over smooth projective varieties are formal.
The Segre cubic S is a simply connected projective threefold with ten singular points, and is described by the set of points of :
A resolution of S is given by the moduli space of stable rational curves with six marked points, and , where is the singular locus of S. We have
Hence S has a non-trivial weight filtration, with . By Theorem 4.5 and since S is simply connected, we may compute the rational homotopy groups with their weight filtration from a minimal model of . Since S is a hypersurface of , the map induces an isomorphism for (see [10, Theorems V.2.6 and V.2.11]). We deduce that
with the only non-trivial products and . The bidegrees are given by , , and . In low degrees we obtain
5 Contractions of subvarieties
Let be a closed immersion of complex projective varieties. Assume that Y contains the singular locus of X and denote by the space obtained by contracting each connected component of Y to a point. In general, is not an algebraic variety. For instance, the contraction of a rational curve in a smooth projective surface is a complex algebraic variety if and only if the self-intersection number of the curve is negative. For contractions of divisors in higher-dimensional varieties there are general conditions on the conormal line bundle, which ensure the existence of contractions in the categories of complex analytic spaces and complex algebraic varieties respectively (see for example [13, 2]). In the general situation in which is a pseudo-manifold with normal isolated singularities, we can endow the rational homotopy type of with a mixed Hodge structure, coming from the mixed Hodge structures on X and Y as follows.
Let be a closed immersion of complex projective varieties. Assume that Y contains the singular locus of X and that X is connected.
The rational homotopy type of carries mixed Hodge structures.
Let be a resolution of X such that is a simple normal crossings divisor. Denote by Σ the singular locus of . There is a string of quasi-isomorphisms from to , where is the bigraded algebra given by
Take mixed Hodge diagrams and with trivial weight filtrations, and let be a mixed Hodge diagram for D, as defined in the proof of Proposition 2.11. Let
By cohomological descent we have . Indeed, since the category of cdgas with the Thom–Whitney simple and the class of quasi-isomorphisms is a cohomological descent category (see [15, Proposition 1.7.4]), it suffices to show that . This follows from excision, since the composition is an isomorphism outside Σ. Hence by Lemma 2.10, is a mixed Hodge diagram for . This proves (1). Assertion (2) follows from Lemma 2.9 and Theorem 2.4. ∎
The weight filtration on the cohomology of a projective variety with isolated singularities has some properties special to the algebraic case, which are not satisfied in the general setting of contractions. For instance, the weight filtration on the cohomology is not semi-pure in general. Another feature of the weight filtration that is only applicable to algebraic varieties is the existence on a non-zero cohomology class , as a consequence of hard Lefschetz theory. Therefore the weight filtration on the cohomology of a contraction may serve as an obstruction theory for such contraction to be an algebraic variety.
Consider two projective lines in intersecting at a point, and let X denote the topological space given by contraction of the two lines to a point in :
Then the weight spectral sequence is given by
Since , the weight filtration on is not semi-pure. Furthermore, we have . This is a proof of the fact that X is not algebraic. Since X is a simply connected topological space of dimension 4, it is formal (see for example [12, Proposition 2.99]). We may compute the rational homotopy groups with their weight filtration from a minimal model of . In low degrees, we have
Let X be a connected complex projective variety of dimension n and let be a closed immersion such that Y contains the singular locus of X. If one of the following conditions is satisfied, then is a formal topological space.
The weight filtration on is pure of weight k , for each .
The link of each singular point in is -connected.
The link of each connected component of Y in X is -connected.
There is a resolution of singularities such that is smooth.
Assume that (a) is satisfied. By Proposition 5.1 (2), the proof of Theorem 3.3 is valid in this setting. Hence purity of the weight filtration implies formality. Note that (b) and (c) are equivalent, since, the links and are homeomorphic (see [11, Application 4.2]). Assume that (b) is satisfied. Let . From the Mayer–Vietoris exact sequence
we find that for , the weight filtration on is pure of weight k. Note that since is not algebraic in general, the weight filtration is not necessarily semi-pure. Hence may have non-trivial weights. The weight spectral sequence for has the form
where the bullets denote non-trivial elements. Consider the quasi-isomorphism of complexes
defined by taking sections of the projections . We next show that ρ is multiplicative. By bidegree reasons, the non-trivial products in are
Since ρ is the identity on , it preserves the former products. It also preserves the unit . Arguing as in the proof of Theorem 4.3, and using the description of given by Proposition 2.13, it is straightforward to see that the latter products are trivial and that the diagram
commutes. We may now proceed as in the proof of Theorem 4.3. Hence the equivalent conditions (b) and (c) imply that is formal. Lastly, assume that (d) is satisfied. The proof of Theorem 4.5 is valid for . Hence is formal. ∎
Let be a closed immersion of smooth projective varieties, with X connected. Then is a formal topological space.
We would like to thank J. W. Morgan for asking us about 1-formality, and A. Dimca for pointing out to us the results on the realization of fundamental groups. Thanks also to F. Guillén and V. Navarro-Aznar for useful comments. J. Cirici acknowledges the Simons Center for Geometry and Physics in Stony Brook at which part of this paper was written.
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About the article
Published Online: 2016-06-18
Published in Print: 2017-01-01
Funding Source: Deutsche Forschungsgemeinschaft
Award identifier / Grant number: SPP-1786
Funding Source: Ministerio de Economía y Competitividad
Award identifier / Grant number: MTM2013-42178-P
The second-named author would like to acknowledge financial support from the German Research Foundation through project SPP-1786 and partial support from the Spanish Ministry of Economy and Competitiveness through project MTM2013-42178-P.