Understanding Groupoids: Axioms & Difference from Groups

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Discussion Overview

The discussion centers around the definition and properties of groupoids, particularly in relation to their axioms and how they differ from groups. Participants explore the foundational aspects of groupoids, including the requirements for binary operations and the implications of these definitions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the axioms that define a groupoid, expressing discomfort with the lack of clarity in a book they are reading.
  • Another participant asserts that a groupoid does not have axioms, describing it simply as any set with a binary operation, providing examples such as (\mathbb{N},+) and (\mathbb{Z},\cdot).
  • It is noted that the binary operation must be well-defined for any two elements in a groupoid, with a follow-up clarification that the set must also be closed under the operation.
  • Participants discuss the distinction between groupoids and groups, with one suggesting that the term "magma" might be more appropriate due to potential confusion with other meanings of groupoid in category theory.

Areas of Agreement / Disagreement

There is no consensus on the axioms of groupoids, as one participant claims there are none while another seeks clarification on the requirements. The discussion reflects differing interpretations of the foundational definitions.

Contextual Notes

Participants mention specific examples of sets and operations that either qualify or do not qualify as groupoids, highlighting the importance of well-defined operations and closure without resolving the broader implications of these definitions.

AdrianZ
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What is a groupoid? I'm reading a book about gyrovectors and in the first chapter it starts defining something which it calls it a groupoid but doesn't inform the reader about its axioms. so I felt uncomfortable to face a new mathematical definition without knowing the axioms that it should satisfy.
Here is what the book says:

Definition 2.1 (Binary Operations, Groupoids, and Groupoid
Automorphisms). A binary operation + in a set S is a function + :
S x S -> S. We use the notation a + b to denote +(a, b) for any a, b in S.
A groupoid (S, +) is a nonempty set, S, with a binary operation, +. An
automorphism phi of a groupoid (S, +) is a bijective (that is, one-to-one)
self-map of S, phi : S -> S, which preserves its groupoid operation, that is,
phi(a + b) = phi(a) + phi(b) for all a, b in S.

What axioms should a groupoid satisfy? and what's the difference between a groupoid and group? because the name apparently has been derived from the word group.
 
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Hi AdrianZ! :smile:

A groupoid doesn't have any axioms. It's just any set with any binary operation.
So (\mathbb{N},+), (\mathbb{N},\cdot), (\mathbb{Z},\cdot), etc are all groupoids.

The only requirement is that the operation must really be an operation. For example (\mathbb{R},/) isn't a groupoid as 1/0 doesn't exist.

A more conventional name for a groupoid is a magma: http://en.wikipedia.org/wiki/Magma_(algebra)
The name magma is preferred, because groupoid often has another meaning in category theory and Lie algebras.
 
So you mean the binary operation must be well-defined for any two elements in a groupoid? right?
 
AdrianZ said:
So you mean the binary operation must be well-defined for any two elements in a groupoid? right?

Yes, the operation must be well-defined and the set must be closed under the operation. (for example (\mathbb{N},-) is not a groupoid since 2-3 is not a natural number. That are the only requirements.

In other words, the operation

*:M\times M\rightarrow M


must be a function.
 
Okay. Thanks.
 

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