Syndetic sensitivity in semiflows

The original Devaney's definition of chaos from 1989 has three conditions, one of which, namely sensitivity, turned out to be redundant. This was first proved by Banks, Brooks, Cairns, Davis and Stacey in 1992. A stronger version of that was proved by Subrahmonian Moothathu in 2007. Another version, independent from Moothathu's, was proved by Wang, Long and Fu in 2012. These statements are corollaries of the main theorem of our paper, which is formulated in the context of an arbitrary acting abelian topological semigroup.