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The graded Euler characteristics have the Jones normalization
Statement
Let be a nonempty finite regular oriented diagram of a link with positive crossings, negative crossings, writhe , and . For each resolution state , let be its number of circles and let use the A-smoothing at coordinates and B-smoothing at coordinates . Put and define the normalized bracket For the unreduced cohomology, define . If is a finite regular oriented diagram of a knot with a basepoint away from all crossings, define its reduced rank Euler characteristic using the reduced quantum grading by . Under and , Thus the unreduced unknot factor is and the reduced unknot factor is .
Statement references
Bracket smoothing weights and loop value
Khovanov state-module and grading conventions
Reduced quotient and quantum shift
Khovanov, A categorification of the Jones polynomial (1999), §§2.2–2.4 and 7.1–7.2
Bar-Natan, On Khovanov's categorification of the Jones polynomial (2002), §§2–3
Kauffman, An Introduction to Khovanov Homology (2022), §§3–4
Facts & Assumptions
Given: A nonempty finite oriented link diagram , its ordered Khovanov state cube, and, when is a based knot diagram, its reduced cochain complex.
The Kauffman state sum is , with each state using the prescribed A/B smoothing chart (Kauffman bracket smoothing and loop value).
The writhe is the sum of crossing signs, so (Oriented crossing signs, diagram writhe, and blackboard framing).
A state with circle labels has cohomological degree and quantum degree ; , , and edge maps preserve (Khovanov Frobenius algebra and cube of resolutions).
The reduced state module kills the based-circle label and subtracts one from the original quantum degree, so the surviving based label contributes degree zero (Reduced Khovanov cohomology of a knot).
The Khovanov procedure returns a bounded bigraded cochain complex with finite-rank free state groups (The Khovanov diagram procedure terminates and returns a finite cochain complex).
Proof
Fix a quantum degree and tensor the finite free complex with . For an integer matrix , the kernel after tensoring is : if with integral and , then because the target is torsion-free. The image after tensoring is by clearing denominators, so rationalized homology is and has dimension . Rank-nullity in this bounded rational complex gives ; alternating over cancels the image dimensions. The same argument applies to the reduced complex, whose terms are finite-rank free.
On a state , summing over its independent labels gives , so the unreduced chain Euler sum is . With , one has and, term by term, . The state contribution is consequently . For the reduced cube, the based label contributes degree zero by [F4], leaving in place of , so its state contribution is .
Summing the state contributions in step 2.1 gives the two displayed formulas for the chain Euler sums. Step 1.1 identifies those sums with the Euler characteristics of integral cohomology. When is the crossingless unknot, and , so the unreduced value is and the reduced value is .
Depends on
- Kauffman bracket smoothing and loop value
- Oriented crossing signs, diagram writhe, and blackboard framing
- Khovanov Frobenius algebra and cube of resolutions
- Khovanov chain complex from a diagram
- Reduced Khovanov cohomology of a knot
- The Khovanov cube differential squares to zero
- Cochain complex in an abelian category
- The Khovanov diagram procedure terminates and returns a finite cochain complex
Used by
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Sources
- Mikhail Khovanov, A categorification of the Jones polynomial, arXiv:math/9908171v2, §§2.2–2.4 and 7.1–7.2 (standard reference, not scraped)
- Dror Bar-Natan, On Khovanov's categorification of the Jones polynomial, Algebraic & Geometric Topology 2 (2002), §§2–3 (standard reference, not scraped)
- Louis H. Kauffman, An Introduction to Khovanov Homology, arXiv:1107.1524v2 (2022), §§3–4 (standard reference, not scraped)
- Melissa Zhang, Notes on Khovanov homology, arXiv:2501.03115v1, §§3.5–3.7 (standard reference, not scraped)