Extremal Transitions from Nested Reflexive Polytopes.

In this paper we look at the question of when an inclusion of reflexive polytopes determines a torically-defined extremal transition between smooth Calabi-Yau hypersurface families. We show this is always possible in dimensions two and three. However, in dimensions four and higher, obstructions can occur. This leads to a smooth projective family of Calabi-Yau threefolds that is birational to one of Batyrev's hypersurface families, but topologically distinct from all such families. [ABSTRACT FROM AUTHOR]