Subset-system derived topologies and coreflective categories of $T_0$-spaces

Abstract

Given a subset system $H$ on $T_0$-spaces, we associate with each topology a closure-testing derived topology and study the fixed points of this operator. The $H$-determined topologies above a given topology are closed under intersections and hence determine a least fixed-point hull. We prove that this hull is obtained by transfinite iteration, is functorial, and defines a right adjoint to the inclusion of $H$-determined spaces into $\mathbf{Top}_0$. Consequently, the corresponding full subcategory is coreflective and complete. We isolate restriction-stability and open-locality hypotheses under which determinedness is inherited by closed and open subspaces, and we establish a general non-reflectivity criterion. If $H$ is contained in the directed-subset system, the category of $H$-determined spaces is Cartesian closed; its exponentials are obtained by coreflecting the pointwise convergence topology on continuous maps. The construction subsumes familiar monotone- and irreducible-determined cases while separating the formal consequences of the subset-system axioms from the additional hypotheses needed for heredity and exponentials. It therefore provides a uniform categorical mechanism for producing complete Cartesian closed categories of $T_0$-spaces with explicit higher-order function spaces, as required in denotational semantics.