The union of two D-spaces need not be D
Dániel T. Soukup, Paul J. Szeptycki (2013)
Fundamenta Mathematicae
Similarity:
We construct from ⋄ a T₂ example of a hereditarily Lindelöf space X that is not a D-space but is the union of two subspaces both of which are D-spaces. This answers a question of Arhangel'skii.