Cohomological shifts of curved differential graded right modules #
The shift by n : โค keeps the underlying right module, replaces the degree-p piece by the
degree-(p + n) piece, and multiplies the differential by (-1)^n. The right action is unchanged.
The sign in the differential compensates for the change of degree in the right Leibniz rule,
and its square is still right multiplication by the same curvature, without a centrality
hypothesis.
On homogeneous cochains of degree p, the DG shift multiplies the underlying map by
(-1)^(n * p). In particular it keeps closed degree-zero maps and negates odd homotopies when
n = 1.
References #
- L. Positselski, Differential graded Koszul duality: an introductory survey, Sections 6.2 and 7.6. His curvature is the negative of the right-module curvature stored here.
The cohomological shift: M[n]แต = Mแตโบโฟ, with differential (-1)^n dM and unchanged
right action and curvature.
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Shifting replaces the degree-p piece by the degree-(p + n) piece.
The differential of the shift acts by the shift sign.
The shift on degree-p cochains, with the sign (-1)^(n * p) that makes it commute with
the Hom differential.
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On underlying maps the cochain shift is multiplication by its Koszul sign.
Shifting cochains commutes with the Hom differential.
The shift by one negates the differential.
The shift leaves degree-zero underlying maps unchanged.
The shift multiplies an odd homotopy by (-1)^n.
The DG shift functor and its induced functors #
The shift preserves identity cochains.
The shift preserves composition of homogeneous cochains.
The action of the shift on homogeneous morphisms of the existing DG category.
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Transport to cochains identifies the DG shift action with the signed cochain shift.
The DG shift action commutes with the differential.
The DG shift action preserves identities.
The DG shift action preserves composition in Mathlib's enriched factor order.
The cohomological shift as a DG endofunctor of curved right modules.
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The DG shift sends a module to its explicit cohomological shift.
The DG shift acts on homogeneous morphisms by the signed cochain shift.
The cohomological shift on the closed degree-zero category of curved modules.
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The closed shift sends a module to its explicit shift.
On closed morphisms the shift keeps the underlying right-linear map.
Zero and addition comparisons in the closed category #
A module shifted by zero is canonically isomorphic to the original module.
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Successive cohomological shifts add their amounts.
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The zero-shift comparison has identity underlying map.
The inverse zero-shift comparison has identity underlying map.
The addition comparison has identity underlying map.
The inverse addition comparison has identity underlying map.
Shifting closed modules by zero is naturally isomorphic to the identity.
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The addition comparisons are natural in closed morphisms.
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The zero comparison is the canonical object comparison.
The inverse zero comparison is the canonical object comparison.
The addition comparison is the canonical object comparison.
The inverse addition comparison is the canonical object comparison.
Transport along equality of shift amounts has identity underlying map.
The cohomological shifts form a coherent integer shift on the closed category.
The cohomological shift on the existing curved homotopy category.
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The homotopy shift sends the class of a module to the class of its explicit shift.
The signed shift of a closed DG morphism is closed.
The homotopy shift maps a represented closed morphism to its shifted representative.
The coherent shift on the curved homotopy category #
The integer shifts descend to the existing homotopy category, with coherence supplied by Mathlib's induced-shift construction.
The quotient functor commutes with the coherent integer shifts.
The quotient-shift comparison is the identity transport on the underlying shifted module, for every integer shift.
Shifting the class of a closed map is the class of its closed shift.
The categorical integer shift is the explicitly constructed homotopy shift.
A categorical shift of a module class is its explicit cohomological shift.