Quotient group
From AoPSWiki
Let 
be a group, and 
an equivalence relation compatible with the group structure on . The structure derived from on the quotient set 
is called the quotient group of by , or the quotient group , or mod .
An equivalence relation 
on is compatible with the group structure on if and only if it is equivalent to a relation of the form 
, for some normal subgroup 
of .
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