# Metric Spaces¶

class sage.categories.metric_spaces.MetricSpaces(category, *args)

The category of metric spaces.

A metric on a set $$S$$ is a function $$d : S \times S \to \RR$$ such that:

• $$d(a, b) \geq 0$$,
• $$d(a, b) = 0$$ if and only if $$a = b$$.

A metric space is a set $$S$$ with a distinguished metric.

Implementation

Objects in this category must implement either a dist on the parent or the elements or metric on the parent; otherwise this will cause an infinite recursion.

Todo

• Implement a general geodesics class.
• Implement a category for metric additive groups and move the generic distance $$d(a, b) = |a - b|$$ there.
• Incorporate the length of a geodesic as part of the default distance cycle.

EXAMPLES:

sage: from sage.categories.metric_spaces import MetricSpaces
sage: C = MetricSpaces()
sage: C
Category of metric spaces
sage: TestSuite(C).run()

class Complete(base_category)

The category of complete metric spaces.

class ElementMethods
abs()

Return the absolute value of self.

EXAMPLES:

sage: CC(I).abs()
1.00000000000000

dist(b)

Return the distance between self and other.

EXAMPLES:

sage: UHP = HyperbolicPlane().UHP()
sage: p1 = UHP.get_point(5 + 7*I)
sage: p2 = UHP.get_point(1 + I)
sage: p1.dist(p2)
arccosh(33/7)

class ParentMethods
dist(a, b)

Return the distance between a and b in self.

EXAMPLES:

sage: UHP = HyperbolicPlane().UHP()
sage: p1 = UHP.get_point(5 + 7*I)
sage: p2 = UHP.get_point(1.0 + I)
sage: UHP.dist(p1, p2)
2.23230104635820

sage: PD = HyperbolicPlane().PD()
sage: PD.dist(PD.get_point(0), PD.get_point(I/2))
arccosh(5/3)

metric()

Return the metric of self.

EXAMPLES:

sage: UHP = HyperbolicPlane().UHP()
sage: m = UHP.metric()
sage: p1 = UHP.get_point(5 + 7*I)
sage: p2 = UHP.get_point(1.0 + I)
sage: m(p1, p2)
2.23230104635820

class SubcategoryMethods
Complete()

Return the full subcategory of the complete objects of self.

EXAMPLES:

sage: Sets().Metric().Complete()
Category of complete metric spaces

class WithRealizations(category, *args)
class ParentMethods
dist(a, b)

Return the distance between a and b by converting them to a realization of self and doing the computation.

EXAMPLES:

sage: H = HyperbolicPlane()
sage: PD = H.PD()
sage: p1 = PD.get_point(0)
sage: p2 = PD.get_point(I/2)
sage: H.dist(p1, p2)
arccosh(5/3)

class sage.categories.metric_spaces.MetricSpacesCategory(category, *args)
classmethod default_super_categories(category)

Return the default super categories of category.Metric().

Mathematical meaning: if $$A$$ is a metric space in the category $$C$$, then $$A$$ is also a topological space.

INPUT:

• cls – the class MetricSpaces
• category – a category $$Cat$$

OUTPUT:

A (join) category

In practice, this returns category.Metric(), joined together with the result of the method RegressiveCovariantConstructionCategory.default_super_categories() (that is the join of category and cat.Metric() for each cat in the super categories of category).

EXAMPLES:

Consider category=Groups(). Then, a group $$G$$ with a metric is simultaneously a topological group by itself, and a metric space:

sage: Groups().Metric().super_categories()
[Category of topological groups, Category of metric spaces]


This resulted from the following call:

sage: sage.categories.metric_spaces.MetricSpacesCategory.default_super_categories(Groups())
Join of Category of topological groups and Category of metric spaces