Double plane of general type Created 2026-09-28 Updated 2026-11-05
Work over an algebraically closed field of characteristic different from two. A smooth projective plane curve of degree is the zero set of a global section of , where . The finite morphism defined locally by is a double cover, branched along . The Jacobian criterion shows that is smooth: away from the derivative in is nonzero, while on some derivative of is nonzero.
The ramification divisor satisfies . The determinant of the differential gives
where is the hyperplane class. Thus is ample for , so is a surface of general type; its self-intersection number is . A smooth octic branch curve gives . For , the same formula instead gives a trivial canonical line bundle of a smooth variety, illustrating why the branch degree changes the classification. The assumption on characteristic is needed for this local double-cover construction and its smoothness argument.
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 gives
and 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, projection
is 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 let
be the degree-two cover branched along . The branch-cover canonical-bundle formula gives
This divisor is ample, so is a double plane of general type and is the required finite morphism of degree two.