Composition in differential graded right-module Hom complexes #
Homogeneous right-module cochains are closed under composition. If g has degree p and f
has degree q, their composite has degree p + q, and the graded-commutator differential obeys
\delta(g \circ f) = \delta(g) \circ f + (-1)^p g \circ \delta(f).
Consequently composition assembles into a morphism of cochain complexes
Hom(N, P) \otimes Hom(M, N) \longrightarrow Hom(M, P).
The order of the tensor factors is Keller's order: the map applied second occurs first. This is also the order for which the tensor-product differential gives the displayed Leibniz rule. The closed composition and unit maps are the algebraic input for the DG category of right modules.
Main definitions #
TauCeti.dgRightModuleCochains.comp: composition of homogeneous right-module cochains.TauCeti.dgRightModuleCochainCompTensor: composition on a pair of homogeneous degrees.TauCeti.dgRightModuleHomComplexComp: composition as a morphism of cochain complexes.TauCeti.dgRightModuleHomComplexUnit: the identity cochain as a morphism from the tensor unit.
References #
- B. Keller, Deriving DG categories, Section 2.
Composition of homogeneous right-module cochains.
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Composition of homogeneous right-module cochains is pointwise composition.
The degree-zero identity cochain of a graded right module.
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Composing on the right with the identity cochain changes nothing.
Composing on the left with the identity cochain changes nothing.
Composition of homogeneous right-module cochains is associative.
The graded commutator satisfies the graded Leibniz rule for composition of cochains, with the sign carried by the degree of the outer factor. Only the degree and the Leibniz rule of the module differentials enter, so this is the Leibniz rule of the Hom differentials of both ordinary and curved differential graded right modules.
The identity cochain is closed for the graded commutator.
The differential on homogeneous right-module cochains satisfies the graded Leibniz rule for composition.
The identity cochain is closed.
Composition on a pair of homogeneous degrees, as a map out of the tensor product of the two cochain modules.
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On a pure tensor, homogeneous composition is ordinary composition of the underlying maps.
Composition of DG right-module cochains is a closed degree-zero map of Hom complexes.
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The identity cochain, as a closed morphism from the tensor unit to the endomorphism Hom complex.
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The degree-zero component of the unit sends a scalar to that scalar multiple of the identity cochain.
Restricting closed composition to a pair of homogeneous summands gives pointwise composition.
Restricting closed composition to a pair of homogeneous summands gives pointwise composition.