Kernel retracts of co-sober spaces

Abstract

Kernel retracts of co-sober spaces need not be co-sober, even when the ambient space is countably infinite and compact. We construct a co-sober $T_0$-space $Y$ and a continuous idempotent map $k:Y\to Y$ satisfying $k(y){\le }_{Y}y$ in the specialization order, whose image with the subspace topology is not co-sober. The construction starts from a countable compact $T_1$ KC anti-Hausdorff space of van Douwen type and adjoins an infinite chain. We classify the $k$-irreducible compact saturated subsets of the extension and show that each is a point saturation. The kernel operator collapses the added chain to a distinguished point while fixing the original space, thereby recovering the non-co-sober seed as its image. This answers negatively two questions of Xu and Zhao, concerning continuous kernel operators and arbitrary retracts, and contrasts with the known preservation of co-sobriety by continuous closure operators.

arXiv comment9 pages