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Feb 23, 2014 at 11:01 comment added diverietti @LiYutong, this is because trivial canonical class implies existence of Ricci flat Kähler metrics (this is Yau), then Kobayashi-Lübke inequalities give you that $c_2(X)\ge 0$ and you have equality if and only if the Ricci flat metric is indeed flat itself. Now you conclude by the classical Bieberbach theorem.
Feb 21, 2014 at 23:23 comment added Li Yutong @diverietti Why "a manifold with trivial canonical class and $c_2(X)=0$ is a finite étale quotient of a complex torus"? Could you explain more about this?
Jul 7, 2011 at 22:49 comment added diverietti did you mean Picard? :p
Jul 7, 2011 at 17:54 comment added Sándor Kovács ps: (of the right Picrad number)
Jul 7, 2011 at 17:53 comment added Sándor Kovács Right. Probably Oguiso-Sakurai meant something like that: Is it true that all CY threefolds admit a rational curve or a non-trivial map from an abelian variety. Perhaps even asking that that map from the abelian variety be dominant.
Jul 7, 2011 at 17:47 comment added diverietti On the other hand, coming back to my original question, these CY threefold with $c_2(X)=0$ certainly admit a non-constant map from a complex torus.
Jul 7, 2011 at 17:44 comment added Sándor Kovács Actually, Oguiso and Sakurai say so themselves right before posing that question "Secondly, our statements (II) and (III) show that there certainly exist smooth Calabi-Yau threefolds containing no rational curves if $\rho(X) = 2$ and $3$, but, on the other hand, suggest some hope to ask the following:" then comes the question quoted by Francesco. Obviously they meant some variation of the question.
Jul 7, 2011 at 17:35 comment added diverietti Thank you for your hints. Anyway, a manifold with trivial canonical class and $c_2(X)=0$ is a finite étale quotient of a complex torus, so that it cannot certainly contain rational curves...
Jul 7, 2011 at 17:30 history answered Francesco Polizzi CC BY-SA 3.0