This entry is about the family of binary relations indexed by the non-negative rational numbers defined by Fred Richman in Real numbers and other completions. For the family of binary relations indexed by the positive rational numbers defined by Auke Booij in Analysis in univalent type theory, see Booij premetric space.
analysis (differential/integral calculus, functional analysis, topology)
metric space, normed vector space
open ball, open subset, neighbourhood
convergence, limit of a sequence
compactness, sequential compactness
continuous metric space valued function on compact metric space is uniformly continuous
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A more general concept of metric space by Fred Richman. While Fred Richman simply called these structures “premetric spaces”, there are multiple notions of premetric spaces in the mathematical literature, so we shall refer to these as Richman premetric spaces.
A Richman premetric space is a set $S$ with a ternary relation $(-)\sim_{(-)}(-)\colon S \times \mathbb{Q}_{\geq 0} \times S \to \Omega$, where $\mathbb{Q}_{\geq 0}$ represent the non-negative rational numbers in $\mathbb{Q}$ and $\Omega$ is the set of truth values, such that
for all $x \in S$ and $y \in S$, $(x = y) \iff (x \sim_0 y)$
for all $x \in S$ and $y \in S$, there exists $q \in \mathbb{Q}_{\geq 0}$ such that $x \sim_q y$
for all $x \in S$, $y \in S$, $q \in \mathbb{Q}_{\geq 0}$, and $r \in (q, \infty)$, where $(q, \infty)$ is the set of all non-negative rational numbers strictly greater than $q$, then $(x \sim_r y) \iff (x \sim_q y)$
for all $x \in S$, $y \in S$, $z \in S$, $q \in \mathbb{Q}_{\geq 0}$, and $r \in \mathbb{Q}_{\geq 0}$, if $x \sim_q y$ and $y \sim_r z$, then $x \sim_{q + r} z$.
Assuming excluded middle, every Richman premetric space is a metric space. Without excluded middle, however, every Richman premetric space is a “metric space” which is valued in the lower Dedekind real numbers, rather than the two-sided Dedekind real numbers.
Last revised on January 28, 2024 at 04:13:54. See the history of this page for a list of all contributions to it.