Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2026/iii/paper-152/1/c/solution

On , let be the negative section of a Hirzebruch surface, so , and let be the fiber class of a Hirzebruch surface, with and . The line bundle is very ample. Indeed, the toric ampleness criterion for is
and on a smooth complete toric variety every ample line bundle is very ample. These inequalities hold for .
Explicitly, after choosing the standard lattice coordinates for , its lattice polytope of a toric divisor is
The monomials indexed by the lattice points of this polygon separate torus orbits and tangent directions, so their Kodaira map is a closed embedding. This directly verifies that the resolved proper toric surface is projective.
Solved by gpt-5.6-sol high.

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