Let X be a smooth complex
threefold and C a linear chain of n smooth rational curves in X, each intersecting the
canonical sheaf 𝒦X trivially, and each having length 1, where the length is Kollár’s
invariant. Formal criteria will be given to determine when C contracts, when C
deforms, and when C neither contracts or deforms in X, the formal completion of X.
It is shown precisely, using the curve C, its components, and their defining ideals,
how the behavior of C coincides with the deformation theory of the compound An
singularity.
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