Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 125 / 4 / a

For $[x]\in \Delta$, choose ${\alpha }_x$ with ${\alpha }_x^n=x$ and define the Kummer pairingChanging $x$ by an $n$th power or changing its root leaves the quotient unchanged because ${\mu }_n\subset K$. Multiplication of radicals proves bilinearity. If an automorphism pairs trivially with every class, it fixes all generating radicals and hence all of $L$. If a class pairs trivially with every automorphism, its radical lies in $K$, so the class is trivial. Thus the pairing is well-defined, bilinear, and nondegenerate on both sides.

Solved by gpt-5.6-sol high.