Equivalently, a simple group is a group possessing exactly two normal subgroups: the trivial subgroup and the group itself. One can also say that a normal subgroup is trivial iff it is not (compare the definition in constructive mathematics below).
Note that the trivial group does not itself count as simple, on the grounds that it has only one quotient group (or only one normal subgroup). It may be possible to find authors that use “at most” in place of “exactly”, thereby allowing the trivial group to be simple. (Compare too simple to be simple.)
In constructive mathematics, we consider a group equipped with a tight apartness such that the group operations are strongly extensional. Then is simple if, given any normal antisubgroup of , owns every nonidentity element (every such that ) iff is inhabited. In other words, is the -complement of the identity subgroup iff is apart from the -complement of the improper subgroup in the sense that the symmetric difference of and is inhabited. (Replacing ‘iff’ with ‘if’ here would allow the trival group to be simple.)
1 = G_0 \subseteq G_1 \subseteq \ldots \subseteq G_n = G
where each inclusion is a normal subgroup and the quotient (called a composition factor) is simple. The condition of simplicity means that that the filtration cannot be further refined by addition of strict inclusions of normal subgroups. Furthermore, the Jordan-Hölder theorem? ensures that any two composition series have the same length and the same composition factors (up to permutation).
Thus finite simple groups are in some sense the primitive building blocks of finite groups generally. The massive program of classifying all finite simple groups was announced as completed by Daniel Gorenstein in 1983, although some doubts remained because there were some gaps in proofs. Most if not all the gaps are considered by experts in the area to have been filled, but there remain some notable skeptics, including for example Jean-Pierre Serre and John H. Conway (verification needed here). See classification of finite simple groups.