Kodaira dimension and the Yamabe problem

The Yamabe invariant Y(M) of a smooth compact manifold is roughly the supre mum of the scalar curvatures of unit-volume constant-scalar-curvature Riema nnian metrics g on M. (To be absolutely precise, one only considers constan t-scalar-curvature metrics which are Yamabe minimizers, but this does not a ffect 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(hl) is c ompletely 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.