Groupoid

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* ''e''<sub>''x''</sub> and ''e''<sub>''x''</sub>' are idempotents, ''i.e.'' ''e''<sub>''x''</sub> * ''e''<sub>''x''</sub> = ''e''<sub>''x''</sub> and ''e''<sub>''x''</sub>' * ''e''<sub>''x''</sub>' = ''e''<sub>''x''</sub>'.
* ''e''<sub>''x''</sub> and ''e''<sub>''x''</sub>' are idempotents, ''i.e.'' ''e''<sub>''x''</sub> * ''e''<sub>''x''</sub> = ''e''<sub>''x''</sub> and ''e''<sub>''x''</sub>' * ''e''<sub>''x''</sub>' = ''e''<sub>''x''</sub>'.
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A groupoid can be seen as:
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*a [[group]] with a [[partial symmetry|partial function]] replacing the [[binary operation]];
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*a category in which every morphism is invertible.
The concept of groupoid as defined here was introduced by Brandt (1927). An alternative meaning of groupoid was introduced by Hausmann and Ore (1937) as a set on which [[binary operation]]s act but neither the identity nor the inversion are included. For this second meaning nowadays the term '''magma''' is used instead (Bourbaki, 1998).
The concept of groupoid as defined here was introduced by Brandt (1927). An alternative meaning of groupoid was introduced by Hausmann and Ore (1937) as a set on which [[binary operation]]s act but neither the identity nor the inversion are included. For this second meaning nowadays the term '''magma''' is used instead (Bourbaki, 1998).

Latest revision as of 13:37, 28 February 2018

Groupoïde (Fr). Gruppoid (Ge). Gruppoide (It). 亜群 (Ja). Группоид (Ru). Grupoide (Sp).


A groupoid (G,*) is a set G with a law of composition * mapping of a subset of G × G into G. The properties of a groupoid are:

  • if x, y, zG and if one of the compositions (x*y)*z or x*(y*z) is defined, so is the other and they are equal (associativity);
  • if x, x' and yG are such that x*y and x' * y are defined and equal, then x = x' (cancellation property);
  • for all xG there exist elements ex (left unit of x), ex' (right unit of x) and x−1 ('inverse' of x) such that:
    • ex * x = x
    • x * ex' = x
    • x−1 * x = ex'.

From these properties it follows that:

  • x * x−1 = ex, i.e. that ex is right unit for x−1,
  • ex' is left unit for x−1
  • ex and ex' are idempotents, i.e. ex * ex = ex and ex' * ex' = ex'.

A groupoid can be seen as:

The concept of groupoid as defined here was introduced by Brandt (1927). An alternative meaning of groupoid was introduced by Hausmann and Ore (1937) as a set on which binary operations act but neither the identity nor the inversion are included. For this second meaning nowadays the term magma is used instead (Bourbaki, 1998).

References

  • Bourbaki, N. (1998). Elements of Mathematics: Algebra 1. Springer.
  • Brandt, H. (1927). Mathematische Annalen, 96, 360-366.
  • Hausmann, B. A. and Ore, O. (1937). American Journal of Mathematics, 59, 983-1004.

See also