Equivalence of metrics

In the study of metric spaces in mathematics, there are various notions of two metrics on the same underlying space being "the same", or equivalent.

In the following, $X$ will denote a non-empty set and $d_{1}$ and $d_{2}$ will denote two metrics on $X$.

Topological equivalence

The two metrics $d_{1}$ and $d_{2}$ are said to be topologically equivalent if they generate the same topology on $X$. The adjective "topological" is often dropped. There are multiple ways of expressing this condition:

• a subset $A\subseteq X$ is $d_{1}$-open if and only if it is $d_{2}$-open;
• the open balls "nest": for any point $x\in X$ and any radius $r>0$, there exist radii $r',r''>0$ such that

$B_{r}(x;d_{1})\subseteq B_{r'}(x;d_{2})$ and $B_{r}(x;d_{2})\subseteq B_{r''}(x;d_{1}).$

• the identity function $I:X\to X$ is both $(d_{1},d_{2})$-continuous and $(d_{2},d_{1})$-continuous.
• there exists a strictly increasing, continuous, and subadditive $f:R_{+}\to R$ such that $d_{2}=f\circ d_{1}$.

The following is a sufficient but not necessary condition for topological equivalence:

• for each $x\in X$, there exist positive constants $\alpha$ and $\beta$ such that, for every point $y\in X$,

$\alpha d_{1}(x,y)\leq d_{2}(x,y)\leq \beta d_{1}(x,y).$

Strong equivalence

Two metrics $d_{1}$ and $d_{2}$ are strongly equivalent if and only if there exist positive constants $\alpha$ and $\beta$ such that, for every $x,y\in X$,

$\alpha d_{1}(x,y)\leq d_{2}(x,y)\leq \beta d_{1}(x,y).$

In contrast to the sufficient condition for topological equivalence listed above, strong equivalence requires that there is a single set of constants that holds for every pair of points in $X$, rather than potentially different constants associated with each point of $X$.

Strong equivalence of two metrics implies topological equivalence, but not vice versa. An intuitive reason why topological equivalence does not imply strong equivalence is that bounded sets under one metric are also bounded under a strongly equivalent metric, but not necessarily under a topologically equivalent metric.

All metrics induced by the p-norm, including the euclidean metric, the taxicab metric, and the Chebyshev distance, are strongly equivalent.

Even if two metrics are strongly equivalent, not all properties of the respective metric spaces are preserved. For instance, a function from the space to itself might be a contraction mapping under one metric, but not necessarily under a strongly equivalent one.

References

• Tensor analysis on manifolds. Dover Publications. 1980. {{cite book}}: Unknown parameter |authors= ignored (help)
• Efe Ok (2007). Real analysis with economics applications. Princeton University Press. ISBN 0-691-11768-3.