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Boundedness of non-birational extremal contractions

Yuri G. Prokhorov
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Abstract

We consider $K_X$-negative extremal contractions $f\colon X\to (Z,o)$, where $X$ is an algebraic threefold with only $ε$-log terminal Q-factorial singularities and $(Z,o)$ is a two (resp., one)-dimensional germ. The main result is that $K_X$ is 1, 2, 3, 4 or 6-complementary or we have, so called, exceptional case and then the singularity $(Z\in o)$ is bounded (resp., the multiplicity of the central fiber $f^{-1}(o)$ is bounded).

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Yuri G. Prokhorov. 1999-06-04. Boundedness of non-birational extremal contractions. https://arxiv.org/abs/math/9901004

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