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A metric space is a pair $(X,d)$, where $X$ is a set and $d:X \times X \to \mathbb{R}$ satisfies the following conditions for all $x,y,z \in X$. (Symmetry) $d(x,y)=d(y,x)$. (Identity of Indiscernibles) $d(x,y)=0$ if and only if $x=y$. (Triangle Inequality) $d(x,y)+d(y,z) \geq d(x,z)$.

5 votes

Are all homogeneous metric spaces bihomogeneous?

Here is a one-dimensional analogue of Richard's triangle example, obtaining a counterexample in the set of reals. Namely, replace every integer $n$ with two numbers at fixed small distance $n\pm\epsil …
Joel David Hamkins's user avatar
11 votes
Accepted

Measure of the support of a Borel probability on a metric space

Following Pietro's lead, let me observe that if there is a measurable cardinal, then there is a counterexample. Suppose that $\kappa$ is a measurable cardinal. Then there is a $\kappa$-additive 2-val …
Joel David Hamkins's user avatar
3 votes
Accepted

Extending a partially defined metric on a metrizable space

Here is a counterexample to Q2, with your stated extra condition. Let $X$ consist of the half-open unit interval $(0,1]$ on the $x$-axis in the plane, together with the full unit interval $[0,1]$ at h …
Joel David Hamkins's user avatar
7 votes

Which metric spaces have this superposition property?

A very natural concept arises if you should insist that $A$ and $B$ are small in some way, such as insisting that they are finite. For example, the countable random graph under the shortest-path metri …
Joel David Hamkins's user avatar
12 votes
Accepted

Axiom of Choice and continuous functions

It seems to me that this is provable without using the axiom of choice. Suppose that $X$ is a compact metric space and $f:X\to\mathbb{R}$ is continuous. Let's show it is uniformly continuous. Fix any …
Joel David Hamkins's user avatar
4 votes

Distance between two points using triangulation

It seems to me that in the general setting of a metric space, what one learns from the sampling data will be precisely the bounds provided by the instances of the triangle inequality that must be obey …
Joel David Hamkins's user avatar