Definition 3.3.2 (Fundamental System of Neighbourhoods).label Let $X$ be a topological space and $A\suf X$. A family $\fraks{B}\suf \cali{N}(A)$ is a fundamental system of neighborhoods/neighborhood base at $A$ if for each $U\in\cali{N}(A)$, there exists $V\in\fraks{B}$ such that $V\suf U$.