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单词 RedundancyOfTwosidednessInDefinitionOfGroup
释义

redundancy of two-sidedness in definition of group


In the definition of group, one usually supposes that there is a two-sided identity elementMathworldPlanetmath and thatany element has a two-sided inverseMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath (cf. group (http://planetmath.org/Group)).

The group may also be defined without the two-sidednesses:

A group is a pair of a non-empty set G and its associative binary operationMathworldPlanetmath(x,y)xy such that

1) the operationMathworldPlanetmath has a right identity element e;

2) any element x of G has a right inverse x-1.

We have to show that the right identity e is also a left identityand that any right inverse is also a left inverse.

Let the above assumptionsPlanetmathPlanetmath on G be true.  If a-1 is the rightinverse of an arbitrary element a of G, the calculation

a-1a=a-1ae=a-1aa-1(a-1)-1=a-1e(a-1)-1=a-1(a-1)-1=e

shows that it is also the left inverse of a.  Using this result,we then can write

ea=(aa-1)a=a(a-1a)=ae=a,

whence e is a left identity element, too.

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更新时间:2025/5/4 10:30:08