Past exam of the mathematics course of the University of Cambridge 2022 iii Paper 112 1 i Solution 2026-09-28
Let be the connected sum of trefoils. The trefoil has Seifert genus one, and additivity of Seifert genus gives
Past exam of the mathematics course of the University of Cambridge 2022 iii Paper 112 1 iv Solution 2026-09-28
Take the untwisted Whitehead double of a trefoil knot. The Whitehead pattern has winding number of a satellite pattern zero and becomes the unknot when its companion is the unknot. The Satellite formula for the Alexander polynomial therefore givesThe Whitehead pattern is geometrically essential in its solid torus, so the satellite knot with nontrivial companion is nontrivial. Hence is not isotopic to the unknot despite having trivial Alexander polynomial of a knot.
Past exam of the mathematics course of the University of Cambridge 2022 iii Paper 112 2 b Solution 2026-09-28
The component knot types can change under this Conway mutation. To construct an example, place a knotted arc carrying a trefoil knot summand in one strand of and a knotted arc carrying a figure-eight knot summand in one strand of , leaving the other two strand portions trivial. Choose the outside pairing so that before rotation the two knotted portions lie on different components, while after rotation they lie on the same component. Then the component multisets arerespectively. A link isotopy preserves the unordered multiset of component knot types, so these links are not isotopic, although part (a) shows that suitable orientations give them the same Jones polynomial.