Definitions
A subgroup, N, of a group, G, is called a normal subgroup if it is invariant under conjugation; that is, for each element n in N and each g in G, the element gng−1 is still in N. We write
For any subgroup, the following conditions are equivalent to normality. Therefore any one of them may be taken as the definition:
- For all g in G, gNg−1 ⊆ N.
- For all g in G, gNg−1 = N.
- The sets of left and right cosets of N in G coincide.
- For all g in G, gN = Ng.
- N is a union of conjugacy classes of G.
- There is some homomorphism on G for which N is the kernel.
The last condition accounts for some of the importance of normal subgroups; they are a way to internally classify all homomorphisms defined on a group. For example, a non-identity finite group is simple if and only if it is isomorphic to all of its non-identity homomorphic images, a finite group is perfect if and only if it has no normal subgroups of prime index, and a group is imperfect if and only if the derived subgroup is not supplemented by any proper normal subgroup.
Read more about this topic: Normal Subgroup
Famous quotes containing the word definitions:
“What I do not like about our definitions of genius is that there is in them nothing of the day of judgment, nothing of resounding through eternity and nothing of the footsteps of the Almighty.”
—G.C. (Georg Christoph)
“Lord Byron is an exceedingly interesting person, and as such is it not to be regretted that he is a slave to the vilest and most vulgar prejudices, and as mad as the winds?
There have been many definitions of beauty in art. What is it? Beauty is what the untrained eyes consider abominable.”
—Edmond De Goncourt (1822–1896)