Neighbourhood system

In topology and related areas of mathematics, the neighbourhood system, complete system of neighbourhoods,^[1] or neighbourhood filter ${\mathcal {N}}(x)$ for a point $x$ in a topological space is the collection of all neighbourhoods of $x.$

^ Mendelson, Bert (1990) [1975]. Introduction to Topology (Third ed.). Dover. p. 41. ISBN 0-486-66352-3.

[1] Mendelson, Bert (1990) [1975]. Introduction to Topology (Third ed.). Dover. p. 41. ISBN 0-486-66352-3.

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