A, where k is considered as a DG-ring concentrated in degree 0; equivalently, a
DG-algebra is a complex of k-modules with a k-linear DG-ring structure.
Remark. One can consider DG-algebras and DG-modules graded by an abelian
group Γ different from Z, provided that Γ is endowed with a parity homomorphism
Γ −→ Z/2 and an odd element 1 Γ, so that the differentials would have degree 1.
In particular, one can take Γ = Z/2, that is have gradings reduced to parities, or
consider fractional gradings by using some subgroup of Q consisting of rationals
with odd denominators in the role of Γ. Even more generally, one can replace the
parity function with a symmetric bilinear form σ : Γ × Γ −→ Z/2, to be used in
the super sign rule in place of the product of parities; one just has to assume that
σ(1, 1) = 1 mod 2. All the most important results of this paper remain valid in
such settings. The only exceptions are the results of subsections 3.4 and 4.3, where
we consider bounded grading.
1.2. DG-categories. A DG-category is a category whose sets of morphisms
are complexes and compositions are biadditive maps compatible with the gradings
and the differentials. In other words, a DG-category DG consists of a class of ob-
jects, complexes of abelian groups HomDG(X, Y ), called the complexes of morphisms
from X to Y , defined for any two objects X and Y , and morphisms of complexes
HomDG(Y, Z) ⊗Z HomDG(X, Y ) −→ HomDG(X, Z), called the composition maps,
defined for any three objects X, Y , and Z. The compositions must be associative
and unit elements idX HomDG(X, X) must exist; the equations d(idX ) = 0 then
hold automatically.
For example, left DG-modules over a DG-ring A form a DG-category, which
we will denote by DG(A–mod). The DG-category of right DG-modules over A will
be denoted by DG(mod–A).
A covariant DG-functor DG −→ DG consists of a map between the classes of
objects and (closed) morphisms between the complexes of morphisms compatible
with the compositions. A contravariant DG-functor is defined in the same way,
except that one has to take into account the natural isomorphism of complexes
V W W V for complexes of abelian groups V and W that is given by
the formula v w −→
v. (Covariant or contravariant) DG-functors
between DG and DG form a DG-category themselves. The complex of morphisms
between DG-functors F and G is a subcomplex of the product of the complexes
of morphisms from F (X) to G(X) in DG taken over all objects X DG ; the
desired subcomplex is formed by all the systems of morphisms compatible with all
morphisms X −→ Y in DG .
For example, a DG-ring A can be considered as a DG-category with a sin-
gle object; covariant DG-functors from this DG-category to the DG-category of
complexes of abelian groups are left DG-modules over A, while contravariant DG-
functors between the same DG-categories can be identified with right DG-modules
over A.
A closed morphism f : X −→ Y in a DG-category DG is an element of
Y ) such that d(f) = 0. The category whose objects are the objects
of DG and whose morphisms are closed morphisms in DG is denoted by Z0(DG).
An object Y is called the product of a family of objects (notation: Y =
Xα) if a closed isomorphism of contravariant DG-functors HomDG(−,Y )
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