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A section of a group is a quotient of a subgroup of . That is, a section of is a group of the form , where is a subgroup of , and is a normal subgroup of .
A group is said to be involved in a group if is isomorphic to a section of .
The relation `is involved in' is transitive, that is, if is involved in and is involved in , then is involved in .
Intuitively, ` is involved in ' means that all of the structure of can be found inside .
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