Are there symplectic 4-folds with $b_+>1$, $b_-=0$? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-21T15:34:04Z http://mathoverflow.net/feeds/question/50569 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/50569/are-there-symplectic-4-folds-with-b-1-b-0 Are there symplectic 4-folds with $b_+>1$, $b_-=0$? Dmitri 2010-12-28T15:30:38Z 2011-01-02T19:13:30Z <p>This is the question. Is it known that a symplectic $4$-fold with $b_2>1$ should have a homology class $C$ with $C^2&lt;0$?</p> http://mathoverflow.net/questions/50569/are-there-symplectic-4-folds-with-b-1-b-0/50838#50838 Answer by Tim Perutz for Are there symplectic 4-folds with $b_+>1$, $b_-=0$? Tim Perutz 2010-12-31T23:45:51Z 2011-01-02T19:13:30Z <p>Symplectic geography in 4 dimensions can be mapped using Chern number coordinates $(c_1^2,c_2)$. The part of the plane where $c_1^2 > 4c_2$ is uncharted. It's unknown whether there are any symplectic 4-manifolds in this region, besides blow-ups of ruled surfaces, though by the Bogomolov-Miyaoka-Yau inequality and the Kodaira-Enriques classification, there are no complex surfaces.</p> <p>I can't answer the question but I'll point out that a symplectic 4-manifold with $b_-=0$ and $b_+>1$ necessarily lies in this unknown region - in particular, it's not Kaehler. </p> <p>To see this, rewrite $c_1^2-4c_2$ in terms of Euler characteristic $\chi$ and signature $\sigma$ as $(2\chi+3\sigma)-4\chi$. For a symplectic manifold with $b_-=0$, this quantity equals $4b_1+b_2-4$ and is positive unless $b_1=0$ and $b_2= 1$ or $3$; I use the parity argument mentioned in Paul's comment. If $b_1=0$ and $b_2=3$ then the intersection form on $H_2/tors.$ is $\mathbb{Z}^3$, the unique rank 3 positive-definite unimodular lattice. So $c_1^2=15$ is the sum of three squares; but it's not. </p>