Kodaira Dimension and the Yamabe Problem

TL;DR

This paper establishes a precise relationship between the Kodaira dimension of a complex algebraic surface and the sign of its Yamabe invariant, linking complex geometry with scalar curvature properties.

Contribution

It demonstrates that the sign of the Yamabe invariant for smooth 4-manifolds from complex surfaces is completely determined by their Kodaira dimension, providing a clear classification.

Findings

Y(M) < 0 iff Kod(M,J)=2

Y(M) = 0 iff Kod(M,J)=0 or 1

Y(M) > 0 iff Kod(M,J)=-infinity

Abstract

The Yamabe invariant Y(M) of a smooth compact manifold is roughly the supremum of the scalar curvatures of unit-volume constant-scalar curvature Riemannian metrics g on M. (To be absolutely precise, one only considers constant-scalar-curvature metrics which are Yamabe minimizers, but this does not affect the sign of the answer.) If M is the underlying smooth 4-manifold of a complex algebraic surface (M,J), it is shown that the sign of Y(M) is completely determined by the Kodaira dimension Kod (M,J). More precisely, Y(M) < 0 iff Kod (M,J)=2; Y(M) = 0 iff Kod (M,J)=0 or 1; and Y(M) > 0 iff Kod (M,J)= -infinity.