Positivity of extensions of vector bundles

Abstract

In this paper, we study when positivity conditions of vector bundles are preserved by extension. We prove that an extension of a big (resp. pseudo-effective) line bundle by an ample (resp. a nef) vector bundle is big (resp. pseudo-effective). We also show that an extension of an ample line bundle by a big line bundle is not necessarily pseudo-effective. In particular, this implies that an almost nef vector bundle is not necessarily pseudo-effective.

v2: 8 pages, completely revised version, to appear in Mathematische Zeitschrift. v1: 7 pages, comments are welcome