Space S000133

Real line with post office metric

Also known as: Real plane with post office metric

A space XX with a particular point pp satisfying:

  • Each point xpx\ne p is isolated.
  • The point pp has a countable neighborhood base {Vn:n<ω}\{V_n:n<\omega\} such that V0=XV_0=X, Vn+1VnV_{n+1}\subseteq V_n and VnVn+1=R|V_n\setminus V_{n+1}|=|\mathbb R| for each n, and n<ωVn={p}\bigcap_{n<\omega}V_n=\{p\}.

These conditions uniquely determine the space XX up to homeomorphism, as shown in Math StackExchange 4833751.

Here are a few examples of spaces satisfying the conditions (see Math StackExchange 4834226).

  • The space X=RX=\mathbb{R} with p=0p=0. The basic open neighborhoods of 00 are the Euclidean open intervals around 00.

  • The post office metric space defined as counterexample #139 ("The Post Office Metric") in DOI 10.1007/978-1-4612-6290-9, where X=RnX=\mathbb R^n with n1n\ge 1 and pp is the origin. The post office metric dd on Rn\mathbb R^n is defined by:

d(x,y)={0if x=y,x+yotherwise.d(x,y) = \begin{cases} 0 & \text{if } x=y, \\ \|x\| + \|y\| & \text{otherwise.} \end{cases}
  • The ordered space (R×Z){}(\mathbb R\times\mathbb Z)\cup\{\infty\} with its order topology, where R×Z\mathbb R\times\mathbb Z has its lexicographical ordering and \infty is an additional maximum point (and the particular point).
Real line with post office metric is a counterexample to the converse of 2 theorems
Id If Then
T65 Weakly locally compactMetrizable Completely metrizable
T269 Weakly locally compactT2T_2Totally disconnected Zero dimensional