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The knot-normalized Alexander fraction need not be a Laurent polynomial
Statement refuted
Assume the full Axiom of Choice and the existing knot convention . The fraction is a Laurent polynomial in for every smooth oriented knot .
Statement references
Facts & Assumptions
Given: Full AC and the standard unknot in the oriented three-sphere.
Full AC is the choice-function axiom used by the Alexander-module and Seifert-matrix conventions. (The Axiom of Choice)
The standard unknot is the oriented unit circle in a fixed coordinate two-plane of . (Oriented connected sum and prime knot)
A Seifert surface is a connected compact smooth oriented embedded surface with the given knot as oriented boundary; a disk has Euler characteristic and genus . (Oriented Seifert surfaces in the three-sphere)
A nonempty contractible space has the singular homology of a point. The singular chain complex of a point has and , so . (Contractible nonempty spaces have the homology of a point)
The positive-push Seifert matrix is defined on an ordered basis of , and the determinant of the empty matrix is . (Seifert pairing and matrix of an oriented knot)
The positive-meridian module is presented by , the square presentation generates by its determinant, and that determinant is associated to . (The one-variable Alexander module of an oriented link, A Seifert matrix presents the one-variable Alexander module, The square Seifert presentation generates the knot Alexander polynomial, The Seifert determinant is the knot Alexander polynomial)
For a knot, the separate knot-normalized fraction is . (The Alexander polynomial from the zeroth elementary ideal)
The assignment extends to a ring homomorphism , since is invertible in the Laurent ring. (The Laurent polynomial ring as the principal localisation of Z[t] at t)
Connected-sum multiplicativity gives . (The knot Alexander polynomial multiplies under connected sum)
Full AC implies the Axiom of Countable Choice, the stated hypothesis of the Seifert-surface definition. (AC implies DC implies countable choice, The Axiom of Countable Choice ())
Every generator satisfies . (Knot Alexander normalization, symmetry, and genus bound)
The Alexander polynomial is unchanged up to a unit under oriented ambient isotopy, and a transported Seifert matrix is related by unimodular congruence. (The Alexander polynomial is an oriented link invariant, Seifert matrices of ambient-isotopic pairs are S-equivalent)
Counterexample
By [A1] and [F9], the choice hypothesis of [F2] holds. The standard unknot bounds a hemisphere disk in a coordinate -sphere in , oriented to induce the chosen orientation of . The disk contracts linearly to its center, so [F3] gives . Its basis is empty, so its Seifert matrix is and its determinant is by [F4]. The matrix theorem and local E0 lemma in [F5] give ; by [F10] choose the representative . By [F11] the computation applies to the oriented unknot type, independently of the chosen representative.
By [F6], . If belonged to , there would be with . Applying [F7] gives in , a contradiction. Thus is not a Laurent polynomial.
By [F8], . Any representative of therefore has the form , whose numerator evaluates to at ; applying [F7] as in step 2.1 shows it is not in .
Depends on
- Seifert pairing and matrix of an oriented knot
- Oriented Seifert surfaces in the three-sphere
- Oriented connected sum and prime knot
- The one-variable Alexander module of an oriented link
- A Seifert matrix presents the one-variable Alexander module
- Seifert matrices of ambient-isotopic pairs are S-equivalent
- The Alexander polynomial from the zeroth elementary ideal
- The Alexander polynomial is an oriented link invariant
- The square Seifert presentation generates the knot Alexander polynomial
- The Seifert determinant is the knot Alexander polynomial
- Knot Alexander normalization, symmetry, and genus bound
- The knot Alexander polynomial multiplies under connected sum
- The Axiom of Choice
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
- AC implies DC implies countable choice
- The Laurent polynomial ring as the principal localisation of Z[t] at t
- Contractible nonempty spaces have the homology of a point
Used by
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Sources
- Dale Rolfsen, Knots and Links (standard reference, not scraped)
- Alessia Cattabriga and Stefano Francaviglia, Knot Theory and Applications: Lecture Notes (standard reference, not scraped)