 K3 surface

A complex K3 surface is a compact, simply connected complex surface with trivial canonical bundle. For projective algebraic surfaces, one can equivalently require a trivial canonical bundle and $H^1(X,{\mathcal{O}}_X)=0$. Nonprojective complex K3 surfaces are also included, which is essential for the full moduli space of Ricci-flat K3 metrics.

A K3 surface is a special type of complex smooth algebraic surface, characterized by several important properties. Here are the key features: 1. **Dimension and Arithmetic**: A K3 surface is a two-dimensional complex manifold (or algebraic surface) with a trivial canonical bundle, meaning that it has a vanishing first Chern class (\(c_1 = 0\)). This implies that its canonical divisor is numerically trivial.