The Cantor-Schreuder-Berstien theorem

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Discussion Overview

The discussion centers around the Cantor-Schreuder-Bernstein theorem and its implications for homomorphisms, particularly in the context of group theory. Participants explore whether the existence of embeddings between two sets implies isomorphism, and the applicability of the theorem to structures that preserve algebraic operations.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that the Cantor-Schreuder-Bernstein theorem guarantees a bijection between two sets if there are one-to-one functions in both directions, but question its implications for homomorphisms.
  • Others argue that the theorem does not imply any structural preservation, stating that it only establishes a bijection without guaranteeing isomorphism.
  • A participant clarifies the definition of homomorphism as a function between algebraic structures that preserves operations.
  • Another participant points out that in the case of group homomorphisms, a version of the Cantor-Schreuder-Bernstein theorem does not hold, citing free groups as a counterexample.

Areas of Agreement / Disagreement

Participants express disagreement regarding the implications of the Cantor-Schreuder-Bernstein theorem for homomorphisms, with some maintaining that it does not imply isomorphism while others explore the conditions under which it might apply.

Contextual Notes

The discussion highlights the limitations of the theorem in the context of algebraic structures, particularly regarding the preservation of operations in group theory. The applicability of the theorem to different types of functions and structures remains unresolved.

gottfried
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The Cantor-Schreuder-Berstien theorem states that if there exists a one-to-one function from X to Y and the reverse then there exists a bijection between X and Y.
Does anybody know if this implies to Homorphisms. ie: If we can find an embedding between X and Y and the reverse does this imply that X and Y are isomorphic?
 
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gottfried said:
The Cantor-Schreuder-Berstien theorem states that if there exists a one-to-one function from X to Y and the reverse then there exists a bijection between X and Y.
Does anybody know if this implies to Homorphisms. ie: If we can find an embedding between X and Y and the reverse does this imply that X and Y are isomorphic?

it implies nothing except that there is a bijection between them. No structure needs to be preserved by the map.
 
What do you mean with "homomorphism" in the first place?
 
What I mean by homomorphism is a function f:(G,.)->(H,*) where f(g.g')=f(g)*f(g')
 
OK, so you're talking about group homomorphisms. Well, in that case, a version of Cantor-Shroder-Bernstein does not hold. A counterexample is given by free groups. Indeed, we can see [itex]F_3[/itex] (free group on three generators) as a subgroup of [itex]F_2[/itex] by considering the subset [itex]\{a^2,ab,b^2\}[/itex] as generators.
 

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