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## balanced curves in Calabi-Yau 3-folds

A balanced smooth rational curve in a calabi-Yau X is a smooth rational curve whose normal bundle is $O(-1)\oplus O(-1)$.

We usually like these curves because of their rigidity.

But, Is there any theorem that guaranty the existence of at least one such curve. For example for Quintic? Or for any other example?

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 As far as I know, there is still no a priori argument known which would prove that a given projective simply-connected Calabi-Yau threefold X contains a rational curve (let alone a balanced one). – Balazs Jun 11 2010 at 14:50

Perhaps you already know this: but we don't even know how to show that there are finitely many rational curves of a given degree $d$ on the general quintic threefold. This was originally conjectured by Clemens. However, for low degrees (up to $d = 11$ or something close to that), the conjecture is verified in the "strong form": any smooth rational curve of low degree (again, at most $11$) on the general quintic has normal bundle $O(-1) \oplus O(-1)$. As far as I know, that's the current state of affairs. It can often be difficult to produce rational curves with the "expected" normal bundle in any given situation!