Threefolds containing all curves are rationally connected (2410.10656v1)

Abstract: Any smooth projective curve embeds into $\mathbb{P}^3$. More generally, any curve embeds into a rationally connected variety of dimension at least three. We prove conversely that if every curve embeds in a threefold $X$, then $X$ is rationally connected. In particular "all curves embed" is a birational property for threefolds.