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چهارشنبه 22 شهریور‌ماه سال 1391

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A group is a pair  (G, *) , where G is a non-empty set and " * '' is binary operation on G, that holds the following conditions.

  • For any  a, b in G , a*b belongs to G. (The operation " * '' is closed).
  • For any  a , b , c in  G ,  (a*b)*c = a*(b*c) . (Associativity of the operation).
  • There is an element   in  G   such that  g*e = e*g = g for any  g in G   . (Existence of identity element).
  • For any in G   there exists an element  h such that  g*h = h*g = e. (Existence of inverses).

Usually, the symbol " * '' is omitted and we write ab for a*b . Sometimes, the symbol " +'' is used to represent the operation, especially when the group is abelian.

It can be proved that there is only one identity element, and that for every element there is only one inverse. Because of this we usually denote the inverse of a as a-1 or –a when we are using additive notation. The identity element is also called neutral element due to its behavior with respect to the operation, and thus a-1 is sometimes (although uncommonly) called the neutralizing element of a.

Groups often arise as the symmetry groups of other mathematical objects; the study of such situations uses group actions. In fact, much of the study of groups themselves is conducted using group actions.

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