*Definition [#ubca43a8]
-Let x be a point of a topological space (X,τ). The essential derived set of x is the set &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b%5c%5b%20%5crm%7bD%7d%5c%7bx%5c%7d=%5crm%7bcl%7d%28%5c%7bx%5c%7d%29%5csetminus%5crm%7bker%7d%28x%29%20%5c%5d%7d%25.png);

*Reference [#o88c43c8]
-Guia, Josep, ''Axioms weaker than R0.'', (Serbo-Croatian summary), Mat. Vesnik 36 (1984), no. 3, 195–205.