If X is a metric space and Y C X, then Y is a metric space.

Elements Of Modern Algebra
8th Edition
ISBN:9781285463230
Author:Gilbert, Linda, Jimmie
Publisher:Gilbert, Linda, Jimmie
Chapter5: Rings, Integral Domains, And Fields
Section5.4: Ordered Integral Domains
Problem 1E: Complete the proof of Theorem 5.30 by providing the following statements, where and are arbitrary...
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Could you explain how to show 9.7 in detail?

Theorem 9.7. If X is a metric space and Y C X, then Y is a metric space.
Definition. A metric on a set M is a function d : M x M →
non-negative real numbers) such that for all a, b, cE M, these properties hold:
R. (where R, is the
(1) d(a, b) > 0, with d(a, b) = 0 if and only if a = b;
(2) d(a, b) = d(b, a);
(3) d(a, c) < d(a, b) + d(b, c).
These three properties are often summarized by saying that a metric is positive defi-
nite, symmetric, and satisfies the triangle inequality.
A metric space (M, d) is a set M with a metric d.
Example. The function d(x, y) = |x – y| is a metric on R. This measure of distance is
the standard metric on R.
%3D
Example. On any set M, we can define the discrete metric as follows: for any a, b e
M, d(a, b) = 1 if a + b and d(a, a) = 0. This metric basically tells us whether two
points are the same or different.
m
Example. Here's a strange metric on Q: for reduced fractions, let d(÷, “) = max(|a –
n
m|, |b – n|). Which rationals are "close" to one another under this metric?
Theorem 9.3. Let d be a metric on the set X. Then the collection of all open balls
В %3 {В(р, є) — {y € X\d(p, у) < e} for every p ЕХаnd every e> 0}
forms a basis for a topology on X.
The topology generated by a metric d on X is called the d-metric topology for X.
Definition. A topological space (X, T) is a metric space or is metrizable if and only
if there is a metric d on X such that J is the d-metric topology. We sometimes write a
metric space as (X, d) to denote X with the d-metric topology.
Theorem 9.6. For any metric space (X, d), there exists a metric d such that d and d
generate the same topology, yet for each x, y E X, d(x,y) < 1.
Transcribed Image Text:Theorem 9.7. If X is a metric space and Y C X, then Y is a metric space. Definition. A metric on a set M is a function d : M x M → non-negative real numbers) such that for all a, b, cE M, these properties hold: R. (where R, is the (1) d(a, b) > 0, with d(a, b) = 0 if and only if a = b; (2) d(a, b) = d(b, a); (3) d(a, c) < d(a, b) + d(b, c). These three properties are often summarized by saying that a metric is positive defi- nite, symmetric, and satisfies the triangle inequality. A metric space (M, d) is a set M with a metric d. Example. The function d(x, y) = |x – y| is a metric on R. This measure of distance is the standard metric on R. %3D Example. On any set M, we can define the discrete metric as follows: for any a, b e M, d(a, b) = 1 if a + b and d(a, a) = 0. This metric basically tells us whether two points are the same or different. m Example. Here's a strange metric on Q: for reduced fractions, let d(÷, “) = max(|a – n m|, |b – n|). Which rationals are "close" to one another under this metric? Theorem 9.3. Let d be a metric on the set X. Then the collection of all open balls В %3 {В(р, є) — {y € X\d(p, у) < e} for every p ЕХаnd every e> 0} forms a basis for a topology on X. The topology generated by a metric d on X is called the d-metric topology for X. Definition. A topological space (X, T) is a metric space or is metrizable if and only if there is a metric d on X such that J is the d-metric topology. We sometimes write a metric space as (X, d) to denote X with the d-metric topology. Theorem 9.6. For any metric space (X, d), there exists a metric d such that d and d generate the same topology, yet for each x, y E X, d(x,y) < 1.
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