Submaximal Riemann-Roch expected curves and symplectic packing

Abstract

We study Riemann–Roch expected curves on $P^1 × P^1$ in the context of the Nagata–Biran conjecture. This conjecture predicts that for a sufficiently large number of points multiple points Seshadri constants of an ample line bundle on algebraic surface are maximal. Biran gives an effective lower bound $N_0$ . We construct examples verifying to the effect that the assertions of the Nagata–Biran conjecture can not hold for small number of points. We discuss cases where our construction fails. We observe also that there exists a strong relation between Riemann–Roch expected curves on $P^1 × P^1$ and the symplectic packing problem. Biran relates the packing problem to the existence of solutions of certain Diophantine equations. We construct such solutions for any ample line bundle on $P^1 × P^1$ and a relatively small number of points. The solutions geometrically correspond to Riemann–Roch expected curves. Finally we discuss in how far the Biran number $N_0$ is optimal in the case of $P^1 × P^1$. In fact, we conjecture that it can be replaced by a lower number and we provide an evidence justifying this conjecture.