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Cancelling a generator with its inverse categorical twist

Example

Fix m≥1 and 1≤i≤m, and let Ri=[ Ui→ βi Am ],Ri−1=[ Am→ γi Ui{−1} ] be the twist complexes of The twist complexes R_i and R_i^{-1}, with Ui in homological degree −1 and Am in degree 0 in the first complex, and Am in degree 0 and Ui{−1} in degree 1 in the second. The totalization N:=Ri⊗AmRi−1 of Signed totalization of graded A_m-bimodule actions is not the diagonal bimodule. Its three nonzero terms are N−1=Ui⊗AmAm≅Ui,N0=(Ui⊗AmUi{−1})⊕(Am⊗AmAm)≅(Ui⊗AmUi{−1})⊕Am,N1=Am⊗AmUi{−1}≅Ui{−1}, so, counted with multiplicity, the tensor complex has the four terms Ui in degree −1, then Ui⊗AmUi{−1} and Am in degree 0, then Ui{−1} in degree 1: it is not concentrated in degree 0. Writing Q:=iP⊗AmPi=Zu1⊕Zu2 with u1=ei⊗ei in degree 0 and u2=(i∣i−1∣i)⊗ei in degree 1, the differentials become the source's maps ∂−1=(τ,βi),∂0(z,a)=γi(a)−δ(z),τ(x⊗y)=x⊗u1⊗(i∣i−1∣i)y+x⊗u2⊗y, δ(x⊗u1⊗y)=x⊗y,δ(x⊗u2⊗y)=x(i∣i−1∣i)⊗y, where the middle tensor coordinate is the negative of the natural balanced identification with Pi⊗ZQ⊗ZiP{−1}. This sign change converts the natural totalization maps (−τ,βi) and (δ,γi) to the source’s displayed chart. The source's splitting of this totalization is N  ≅  T−1⊕Am⊕T1, where Am is the diagonal bimodule in degree 0, and T−1,T1 are the two two-term complexes T−1=[ Ui→ ∂−1 ∂−1(Ui) ],T1=[ Pi⊗ZZu1⊗ZiP{−1}→ −δ Ui{−1} ], whose differentials are invertible; T−1 and T1 are therefore contractible, with contracting homotopies the inverses (∂−1)−1 and x⊗y↦−x⊗u1⊗y of the displayed differentials. Thus the inverse pair cancels up to homotopy, not on the nose: the two contractible summands are the visible cost of the cancellation. The same holds with the factors in the opposite order.

Facts & Assumptions

Given: An integer m≥1, an index 1≤i≤m, the complexes Ri,Ri−1 with the maps βi,γi, the bimodule Ui and its internal shift, the corner basis eiAmei=Zei⊕Z(i∣i−1∣i), and the totalization of Signed totalization of graded A_m-bimodule actions.

[L1]

Ri=[Ui→βiAm] with Ui in degree −1 and Am in degree 0, and Ri−1=[Am→γiUi{−1}] with Am in degree 0 and Ui{−1} in degree 1; both are bounded complexes of graded bimodules with two-sided finite graded projective terms and degree-zero differentials (The twist complexes R_i and R_i^{-1}).

[L2]

Ri⊗AmRi−1≃Am and Ri−1⊗AmRi≃Am via homotopy equivalences; the proof exhibits the splitting of the first tensor complex and the explicit maps τ,δ,ξ (The generator complexes are mutually inverse).

[L3]

βi(ei⊗ei)=ei and γi(1)=wi with wi the four-term sum displayed in the Definition; τ and δ are the degree-zero bimodule maps with δτ=γiβi, δξ=γi, and the square (2.8) anticommutes (The Khovanov–Seidel bimodule maps β_i and γ_i, The generator complexes are mutually inverse).

[L4]

The totalization of two bounded complexes of graded bimodules has the terms (R⊗AmS)n=⨁p+q=nRp⊗AmSq and the differential d(r⊗s)=dRr⊗s+(−1)pr⊗dSs, and the tensor-unit maps M⊗AmAm≅M, Am⊗AmM≅M are canonical degree-zero isomorphisms (Signed totalization of graded A_m-bimodule actions).

[L5]

A two-term complex with invertible differential is contractible, with the inverse differential as contracting homotopy (Gaussian elimination splits a contractible two-term complex).

Verification

technique · direct
1.1L1L4

The four terms and their degrees. By [L4] the terms of N=Ri⊗AmRi−1 are the direct sums over p+q=n of Rip⊗Am(Ri−1)q; the nonzero pairs are (p,q)=(−1,0),(0,0),(−1,1),(0,1), giving the terms Ui, Am, Ui⊗AmUi{−1} and Ui{−1} in homological degrees −1,0,0,1 as displayed, with the two degree-0 summands ordered as Ui⊗AmUi{−1} and Am; the tensor-unit isomorphisms of [L4] identify the first and last with Ui and Ui{−1}. This corrects the term count: there are four terms counted with multiplicity, not three Am's.

2.1step 1.1L3L4

The differentials. After negating the natural identification of Ui⊗AmUi{−1} with Pi⊗ZQ⊗ZiP{−1} and the tensor-unit identifications, the Koszul-signed differentials of [L4] take the form ∂−1(u)=(τ(u),βi(u)) and ∂0(z,a)=γi(a)−δ(z) with the maps τ,δ of the display: the component into Am is βi, the component into the middle bimodule is τ, and the component out of Am is γi, while the component out of the middle is −δ after this coordinate change. Before that change the Koszul rule gives (−τ,βi) and (δ,γi), as required.

3.1step 2.1L3L5

The two contractible summands. The u2 component of τ is the identity on Ui, so ∂−1 is injective and its restriction to its image is an isomorphism with inverse the u2-coefficient projection on the middle summand, so T−1 is contractible with contracting homotopy (∂−1)−1 by [L5]; the restriction −δ ⁣:Pi⊗Zu1⊗iP{−1}→Ui{−1} is an isomorphism with inverse x⊗y↦−x⊗u1⊗y, so T1 is contractible by [L5]. The map a↦(ξ(a),a) identifies the remaining graph in degree 0 with Am with zero differential, because ∂0(ξ(a),a)=γi(a)−δξ(a)=0.

4.1step 3.1L2∎

Conclusion of the example. By the direct sum decomposition of the generator lemma [L2], N≅T−1⊕Am⊕T1; by step 3.1 the outer summands are contractible, so N is homotopy equivalent to the diagonal bimodule and RiRi−1≅Id⁡Cm, while N itself is a four-term complex and not equal to Am; the same argument applies to Ri−1⊗AmRi. The two contractible summands are exactly the cancellation cost, and their contracting homotopies are the displayed inverses.

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