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Mathlib.Algebra.Homology.TotalComplexShift

Behaviour of the total complex with respect to shifts #

There are two ways to shift objects in HomologicalComplex₂ C (up ℤ) (up ℤ):

These two sorts of shift functors shall be abbreviated as HomologicalComplex₂.shiftFunctor₁ C x and HomologicalComplex₂.shiftFunctor₂ C y.

In this file, for any K : HomologicalComplex₂ C (up ℤ) (up ℤ), we define an isomorphism K.totalShift₁Iso x : ((shiftFunctor₁ C x).obj K).total (up ℤ) ≅ (K.total (up ℤ))⟦x⟧ for any x : ℤ (which does not involve signs) and an isomorphism K.totalShift₂Iso y : ((shiftFunctor₂ C y).obj K).total (up ℤ) ≅ (K.total (up ℤ))⟦y⟧ for any y : ℤ (which is given by the multiplication by (p * y).negOnePow on the summand in bidegree (p, q) of K).

Depending on the order of the "composition" of the two isomorphisms totalShift₁Iso and totalShift₂Iso, we get two ways to identify ((shiftFunctor₁ C x).obj ((shiftFunctor₂ C y).obj K)).total (up ℤ) and (K.total (up ℤ))⟦x + y⟧. The lemma totalShift₁Iso_trans_totalShift₂Iso shows that these two compositions of isomorphisms differ by the sign (x * y).negOnePow.

The isomorphism (((shiftFunctor₁ C x).obj K).X a).X b ≅ (K.X a').X b when a' = a + x.

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    The isomorphism (((shiftFunctor₂ C y).obj K).X a).X b ≅ (K.X a).X b' when b' = b + y.

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      Auxiliary definition for totalShift₁Iso.

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        The isomorphism ((shiftFunctor₁ C x).obj K).total (up ℤ) ≅ (K.total (up ℤ))⟦x⟧ expressing the compatibility of the total complex with the shift on the first indices. This isomorphism does not involve signs.

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          Auxiliary definition for totalShift₂Iso.

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            The isomorphism ((shiftFunctor₂ C y).obj K).total (up ℤ) ≅ (K.total (up ℤ))⟦y⟧ expressing the compatibility of the total complex with the shift on the second indices. This isomorphism involves signs: on the summand in degree (p, q) of K, it is given by the multiplication by (p * y).negOnePow.

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