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Cancelling a generator with its inverse categorical twist
Example
Fix and , and let be the twist complexes of The twist complexes R_i and R_i^{-1}, with in homological degree and in degree in the first complex, and in degree and in degree in the second. The totalization of Signed totalization of graded A_m-bimodule actions is not the diagonal bimodule. Its three nonzero terms are so, counted with multiplicity, the tensor complex has the four terms in degree , then and in degree , then in degree : it is not concentrated in degree . Writing with in degree and in degree , the differentials become the source's maps where the middle tensor coordinate is the negative of the natural balanced identification with . This sign change converts the natural totalization maps and to the source’s displayed chart. The source's splitting of this totalization is where is the diagonal bimodule in degree , and are the two two-term complexes whose differentials are invertible; and are therefore contractible, with contracting homotopies the inverses and 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 , an index , the complexes with the maps , the bimodule and its internal shift, the corner basis , and the totalization of Signed totalization of graded A_m-bimodule actions.
with in degree and in degree , and with in degree and in degree ; 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}).
and via homotopy equivalences; the proof exhibits the splitting of the first tensor complex and the explicit maps (The generator complexes are mutually inverse).
and with the four-term sum displayed in the Definition; and are the degree-zero bimodule maps with , , and the square (2.8) anticommutes (The Khovanov–Seidel bimodule maps β_i and γ_i, The generator complexes are mutually inverse).
The totalization of two bounded complexes of graded bimodules has the terms and the differential , and the tensor-unit maps , are canonical degree-zero isomorphisms (Signed totalization of graded A_m-bimodule actions).
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
The four terms and their degrees. By [L4] the terms of are the direct sums over of ; the nonzero pairs are , giving the terms , , and in homological degrees as displayed, with the two degree- summands ordered as and ; the tensor-unit isomorphisms of [L4] identify the first and last with and . This corrects the term count: there are four terms counted with multiplicity, not three 's.
The differentials. After negating the natural identification of with and the tensor-unit identifications, the Koszul-signed differentials of [L4] take the form and with the maps of the display: the component into is , the component into the middle bimodule is , and the component out of is , while the component out of the middle is after this coordinate change. Before that change the Koszul rule gives and , as required.
The two contractible summands. The component of is the identity on , so is injective and its restriction to its image is an isomorphism with inverse the -coefficient projection on the middle summand, so is contractible with contracting homotopy by [L5]; the restriction is an isomorphism with inverse , so is contractible by [L5]. The map identifies the remaining graph in degree with with zero differential, because .
Conclusion of the example. By the direct sum decomposition of the generator lemma [L2], ; by step 3.1 the outer summands are contractible, so is homotopy equivalent to the diagonal bimodule and , while itself is a four-term complex and not equal to ; the same argument applies to . The two contractible summands are exactly the cancellation cost, and their contracting homotopies are the displayed inverses.
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