Approach space

2020 Mathematics Subject Classification: Primary: 54A05 Secondary: 54E05 [MSN][ZBL]

A generalisation of the concept of metric space, formalising the notion of the distance from a point to a set. An approach space is a set $X$ together with a function $d$ on $X \times \mathcal{P}X$, where $\mathcal{P}X$ is the power set of $X$, talking values in the extended positive reals $[0,\infty]$, and satisfying $$ d(x,\{x\}) = 0 \ ; $$ $$ d(x,\emptyset) = \infty \ ; $$ $$ d(x,A\cup B) = \min(d(x,A),d(x,B)) \ ; $$ $$ d(x,A) \le d(x,A^u) + u \ ; $$ where for $u \in [0,\infty]$, we write $A^u = \{x \in X : d(x,A) \le u \}$.

A metric space $(X,\delta)$ has an approach structure via $$ d(x,A) = \inf\{ \delta(x,a) : a \in A \} \ . $$ and a topological space $(X,{}^c)$, where ${}^c$ denotes the Kuratowski closure operator, via $$ d(x,A) = \begin{cases} 0 & \ \text{if}\ x \in A^c \\ \infty & \ \text{otherwise} \end{cases} \ . $$

In the opposite direction, if $(X,d)$ is an approach space then the operation $$ A^c = \{ x \in X : d(x,A) < \infty \} $$ is a Čech closure operator, giving $X$ the structure of a pre-topological space. However, the operation $$ A^C = \{ x \in X : d(x,A) = 0 \} $$ is a closure operator giving a topological structure on $X$.

References

• Hofmann, Dirk (ed.); Seal, Gavin J. (ed.); Tholen, Walter (ed.) "Monoidal topology. A categorical approach to order, metric, and topology" Cambridge University Press (2014) ISBN 978-1-107-06394-5 Zbl 1297.18001
• R. Lowen, "Approach spaces - a common supercategory of TOP and MET." Math. Nachr. 141 (1989) 183-226 Zbl 0676.54012
• R. Lowen, "Index Analysis: Approach Theory at Work", Springer (2015) ISBN 1-4471-6485-7 Zbl 1311.54002