Set theory in Munkres Topology

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Discussion Overview

The discussion centers on the definition of the Cartesian product in set theory as presented in Munkres' Topology. Participants explore the implications of different definitions and the importance of ordered pairs in distinguishing between elements.

Discussion Character

  • Conceptual clarification, Debate/contested, Technical explanation

Main Points Raised

  • One participant questions the necessity of defining the Cartesian product as {{a},{a,b}} instead of {{a},{b}}, suggesting that both yield the same result when a = b.
  • Another participant emphasizes the importance of distinguishing between ordered pairs (a,b) and (b,a), noting that sets do not inherently possess order, which necessitates the specific definition used by Munkres.
  • A later reply confirms the distinction between (a,b) and (b,a) using the definitions provided, illustrating that the proposed definition would lead to ambiguity in identifying ordered pairs.
  • Participants discuss the intersection and union of the sets representing (a,b) and (b,a), noting the resulting sets from these operations.
  • A light-hearted comment about coffee is made, suggesting a humorous take on the discussion's intensity.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of distinguishing ordered pairs in the context of Cartesian products, but there is disagreement regarding the sufficiency of alternative definitions proposed.

Contextual Notes

The discussion does not resolve the implications of using different definitions for the Cartesian product, nor does it address potential limitations in the definitions themselves.

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In Munkres' Topology he defines a Cartesian product AxB to be all (a,b) such that a is in A and b is in B. He says that this is a primative way of looking at things. And then defines it to be {{a},{a,b}}

He says that if a = b then {a,b} will just be {a,a} = {a} and therefore will only be {{a}}.

What I don't understand is the the need for {a,b}, why not just define the Cartesian product to be {{a},{b}}. If a = b you get the same result.
 
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You want to distinguish between (a,b) and (b,a). Your idea does not do this.

The key thing is the notion of ordering. Sets do not come with any order on the elements. This is why you need this fiddle if you wish to define the cartesian product of sets purely in set theoretic terms.
 
Thanks so much!

So (a,b) = {{a},{a,b}} and (b, a) = {{b}, {b,a}}

Clearly (a,b) != (b,a)

Where as if you use my proposed definition you would have...

(a,b) = {{a},{b}} and (b,a) = {{b}, {a}}, but these are the same sets.

What is interesting is to me now is looking at the intersection and union of (a,b) and (b,a)

The intersection is {{a,b}}, the union is {{a},{b},{a,b}}. Cool!

On a side note Matt Grime, I don't know the correct quote in you signature, but you are clearly missing another essential tool of a mathematician... Coffee.
 
That's what I'm doing wrong; I don't drink coffee! :-p
 

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