Let be a smooth connected ample curve in an abelian surface . Translate its chosen basepoint to zero and apply the universal property of the Jacobian variety. The image of is a closed connected algebraic subgroup containing the translated curve. If it had dimension one, that curve would be the subgroup itself; a distinct coset is disjoint and numerically equivalent, forcing . This contradicts ampleness, so the homomorphism is surjective.
Past exam of the mathematics course of the University of Cambridge 2008 iii Paper 25 4 Solution Created 2026-10-03 Updated 2026-10-14
Choose a projective embedding of the abelian surface . By Bertini's theorem, a general hyperplane section is a smooth projective curve that is connected. It is ample, so . Choose and translate the inclusion so it sends to the identity of . The universal property of the Jacobian variety then gives a homomorphism of abelian varietieswhose composition with the based Abel map of an algebraic curve is this translated inclusion.
The image is a closed connected algebraic subgroup, since is projective, and it contains a translate of . If the image had dimension one, that irreducible curve would be the image subgroup itself. A distinct coset is disjoint from it and has the same numerical divisor class: translation acts trivially on cohomology, being homotopic to the identity on the complex torus. Its self-intersection would therefore be zero, contradicting . The image has dimension two and is all of . Thus is surjective, as expressed by the principle that an ample curve on an abelian surface generates the surface.
Past exam of the mathematics course of the University of Cambridge 2021 iii Paper 159 2 Solution Created 2026-09-28 Updated 2026-10-14
A minimal algebraic surface is a smooth projective surface containing no exceptional curve of the first kind, namely no smooth rational curve of self-intersection . An abelian surface contains no rational curve because every morphism from to an abelian variety is constant. It therefore has no -curve and is minimal.
For every , the Minimal Hirzebruch surface is a rational minimal surface. These surfaces are pairwise nonisomorphic: the negative section is the unique irreducible curve of negative self-intersection and has square , so an isomorphism would recover . Thus there are infinitely many nonisomorphic minimal rational surfaces.
A K3 surface is a smooth projective surface with and . For a smooth curve of geometric genus , the adjunction formula givesand hence
An elliptic surface is a smooth projective surface with a morphism to a smooth curve whose generic fiber is a smooth genus-one curve. If is an elliptic curve, projectionis an elliptic fibration. Its canonical bundle is pulled back from , so every positive pluricanonical space vanishes and the Kodaira dimension is . This supplies the requested negative-Kodaira-dimension example.
For an elliptically fibered K3 surface, choose a smooth quartic containing a line . The canonical bundle of a smooth projective hypersurface formula makes trivial, and the standard cohomology sequence gives , so is K3. The pencil of planes through cuts into plus a residual plane cubic. The residual linear system is basepoint-free, has square zero and defines a morphism whose generic fiber is a smooth plane cubic. This is the Elliptic K3 surface from a quartic containing a line.
A surface of general type is a smooth projective surface of Kodaira dimension two. Let be a smooth plane curve of degree eight and letbe the degree-two cover branched along . The branch-cover canonical-bundle formula givesThis divisor is ample, so is a double plane of general type and is the required finite morphism of degree two.