Английская Википедия:CEP subgroup
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In mathematics, in the field of group theory, a subgroup of a group is said to have the Congruence Extension Property or to be a CEP subgroup if every congruence on the subgroup lifts to a congruence of the whole group. Equivalently, every normal subgroup of the subgroup arises as the intersection with the subgroup of a normal subgroup of the whole group.
In symbols, a subgroup <math>H</math> is a CEP subgroup in a group <math>G</math> if every normal subgroup <math>N</math> of <math>H</math> can be realized as <math>H \cap M</math> where <math>M</math> is normal in <math>G</math>.
The following facts are known about CEP subgroups:
- Every retract has the CEP.
- Every transitively normal subgroup has the CEP.
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