Toric multisections and curves in rational surfaces

Abstract

We study multisections of embedded surfaces in 4-manifolds admitting effective torus actions. We show that a simply-connected 4-manifold admits a genus one multisection if and only if it admits an effective torus action. Orlik and Raymond showed that these 4-manifolds are precisely the connected sums of copies of $\mathbb{CP}^2$, $\overline{\mathbb{CP}^2}$, and $S^2\times S^2$. Therefore, embedded surfaces in these 4-manifolds can be encoded diagrammatically on a genus one surface. Our main result is that every smooth, complex curve in $\mathbb{CP}^1\times\mathbb{CP}^1$ can be put in efficient bridge position with respect to a genus one 4-section. We also analyze the algebraic topology of genus one multisections.