Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2026/iii/paper-112/2/d/solution

For the supplied Seifert matrix ,
and the symmetric form has signature . Thus
Since the Levine-Tristram signature is an additive homomorphism on the algebraic concordance group, has infinite algebraic-concordance order.
Over , reduction of the Alexander polynomial gives
The factors are coprime, nonsymmetric, and exchanged by reciprocity. Hensel lifting therefore decomposes the local isometric structure into a reciprocal pair, which is metabolic. Its class in is zero and in particular does not have order four.
For , diagonalization gives
The second residue at is the one-dimensional form
Because , and this one-dimensional form is a generator. The P-adic algebraic-concordance obstruction therefore has exact order four, so the image of in has order four.
Solved by gpt-5.6-sol high.

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