设盖是射影流形,f:X→Y是X的小收缩态射,f的例外集E是光滑子簇.如果f(E)是零维的,E的维数不大于X的一半且法丛NE/X与+OE(-1)同构,t=codimE,那么f的翻转f^+:X^*→Y一定存在.
Let X be a projective manifold andf:X→ Y a small contraction morphism of X. Let the exceptional locus E off be smooth subvariety. Suppose that the dimension off(E) is zero and the normal bundleis NE/x is +OE( -1 ) ,where t = codimE. Then there exists the flip f+ :X+→Y of f.