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A prime-power torsion linking form need not have a metabolizer
Statement refuted
Assume AC. The assertion that every nonsingular linking form on a finite group of odd prime-power order has a metabolizer is false: the double branched cover of the positive trefoil has cyclic first homology of order three and no metabolizer.
Statement references
Facts & Assumptions
Given: The positive trefoil, its Seifert matrix , and the double branched-cover orientation convention.
Full AC is assumed wherever the cover and orientation conventions use the choice axiom. The Axiom of Choice
The double-cover surgery matrix is . Surgery presentation from a Seifert surface
The oriented linking form on the surgery cokernel is . Inverse-matrix formula for a surgery linking form
A finite nonsingular linking form has a metabolizer only if some subgroup has square-root order. Finite nonsingular linking forms and metabolizers
The surgery linking matrix computes the same oriented torsion linking pairing as the signed bounding-chain formula. Bounding-chain expression for a torsion linking pairing Inverse-matrix formula for a surgery linking form
A closed oriented -manifold with finite first homology has nonsingular torsion linking form. The torsion linking pairing of a rational homology sphere is nonsingular
Counterexample
Computing gives with determinant , and unimodular row and column operations reduce it to . Therefore . The matrix formula [F3] gives generator self-linking under [F5], so the full group is not isotropic.
The only subgroups of have order or , while a metabolizer would have order by [F4]. The form is nonsingular by [F6] and step 1.1, so it satisfies the assertion's nonsingularity hypothesis. No subgroup has that order, so no metabolizer exists. This finite nonsingular form of odd prime-power order refutes the assertion.
Depends on
- Bounding-chain expression for a torsion linking pairing
- Finite nonsingular linking forms and metabolizers
- The torsion linking pairing of a rational homology sphere is nonsingular
- Surgery presentation from a Seifert surface
- Inverse-matrix formula for a surgery linking form
- A positive Hopf plumbing gives the trefoil Seifert matrix
- The Axiom of Choice
Used by
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Sources
- Dale Rolfsen, Knots and Links (standard reference, not scraped)
- Jozef H. Przytycki and Akira Yasuhara, Linking Numbers in Rational Homology 3-Spheres, Cyclic Branched Covers and Infinite Cyclic Covers (standard reference, not scraped)