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Algebraically closed group : ウィキペディア英語版
Algebraically closed group
In mathematics, in the realm of group theory, a group A\ is algebraically closed if any finite set of equations and inequations that "make sense" in A\ already have a solution in A\ . This idea will be made precise later in the article.
==Informal discussion==

Suppose we wished to find an element x\ of a group G\ satisfying the conditions (equations and inequations):
::x^2=1\
::x^3=1\
::x\ne 1\
Then it is easy to see that this is impossible because the first two equations imply x=1\ . In this case we say the set of conditions are inconsistent with G\ . (In fact this set of conditions are inconsistent with any group whatsoever.)
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Now suppose G\ is the group with the multiplication table:
Then the conditions:
::x^2=1\
::x\ne 1\
have a solution in G\ , namely x=a\ .
However the conditions:
::x^4=1\
::x^2a^ = 1\
Do not have a solution in G\ , as can easily be checked.
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|b \
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|a \
|1 \
|c \
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|b \
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However if we extend the group G \ to the group H \ with multiplication table:
Then the conditions have two solutions, namely x=b \ and x=c \ .
Thus there are three possibilities regarding such conditions:
* They may be inconsistent with G \ and have no solution in any extension of G \ .
* They may have a solution in G \ .
* They may have no solution in G \ but nevertheless have a solution in some extension H \ of G \ .
It is reasonable to ask whether there are any groups A \ such that whenever a set of conditions like these have a solution at all, they have a solution in A \ itself? The answer turns out to be "yes", and we call such groups algebraically closed groups.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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