Properties of a group
How can we say that the pair (G, o) is a group. Explain the properties which proof it.
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Let G be a set and suppose that o is a binary operation on G. We say that the pair (G; o) is a group if it has the following properties.
(i) The operation o is associative; that is, (g o h) o k = g o (h o k) for all g; h; k ≡ G.(ii) There exists an identity for o ; that is, there exists e ≡ G such that g o e = e o g = g for all g ≡ G.(iii) There exist inverses for o ; that is, for each g ≡ G, there exists g-1≡ G such thatg o g-1 = g-1 o g = e:There is another property implicit in this denition which it is useful to give a name to. Instead of saying that o is a binary operation on G, we can say that the law of closure holds for o, meaning that when o acts on two elements of G the result is also in G.
Most of the groups (G; o) we study will also have the following property.
(iv) The operation o is commutative; that is, g o h = h o g for all g; h ≡ G.
A group with this property is called commutative or, more usually, abelian, after the Norwegian mathematician Niels Henrik Abel (1802{1829).
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