Definition
- A topological space (X, τ) is called an E_1 space if for x ∈ X , where C_n^(x) is a closed neighborhood of x.
Reference
- Aull, C. E., "Some base axioms for topology involving enumerability.", 1971 General Topology and its Relations to Modern Analysis and Algebra, III (Proc. Conf., Kanpur, 1968) pp. 55-61.