Definition of Path: Are the Two Definitions Equivalent?

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Discussion Overview

The discussion revolves around the definitions of a path in a metric space, specifically comparing two definitions: one that allows for arbitrary endpoints \(a\) and \(b\) and another that standardizes the endpoints to \(0\) and \(1\). The scope includes theoretical aspects of real analysis and the implications of these definitions in mathematical contexts.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant cites a definition of a path as a continuous function \(f: [a,b] \to M\) with endpoints \(f(a) = p\) and \(f(b) = q\), while another presents a definition using \(f: [0,1] \to M\) with \(f(0) = p\) and \(f(1) = q\).
  • Another participant argues that the definitions are not equivalent in a literal sense, noting that paths defined by the first definition can exist that do not fit the second definition if \(a\) and \(b\) are not \(0\) and \(1\).
  • However, this participant also suggests that the practical applications of paths may not be affected by this difference, implying that the distinction may be largely theoretical.
  • A subsequent reply seeks clarification on whether the endpoints \(a\) and \(b\) can be specifically chosen as \(0\) and \(1\) in the context of the first definition.
  • Another participant proposes a method to convert a path defined on \([a,b]\) into one defined on \([0,1]\) by using a linear transformation, indicating a potential way to reconcile the two definitions.

Areas of Agreement / Disagreement

Participants express disagreement regarding the equivalence of the two definitions, with some acknowledging the theoretical distinction while others suggest that the practical implications may not differ significantly. The discussion remains unresolved regarding the necessity of distinguishing between the two definitions in applications.

Contextual Notes

The discussion highlights the dependence on the choice of endpoints in defining paths and the implications of this choice on the equivalence of definitions. There is also an assumption that the transformation method proposed is valid without further exploration of its implications.

JG89
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My real analysis book says that a path from two points p, q in a metric space M is a continuous function f: [a,b] --> M such that f(a) = p and f(b) = q, for some a and some b. But when I read other definitions, it says a path from two points p, q in a metric space M is a continuous function f: [0,1] ---> M such that f(0) = p, f(1) = q.

Are the two definitions equivalent?
 
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Yes and no.

In the most literal sense, they are clearly inequivalent -- there exist paths by the first definition that obviously aren't paths by the second definition. (just choose (a,b) to be anything but (0,1))


However, I can't think of any application of paths that really cares about the difference between the two definitions.


(note that you can always compose with your favorite order isomorphism [0,1]-->[a,b] to convert any path [a,b]-->M into a path [0,1]-->M)
 
ahhh, are you saying a=0, b=1 for your some a,b?
 
if f : [a,b] --> M , g : [0,1] ---> M. and you want f(a) = g(0), and f(b) = g(1).

let g(x) = f(a + (b-a)x).
 

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