Definition A subgroup series for a group G is a chain of normal subgroups that gets down to the trivial group 1=G0≤G1≤G2≤…≤Gn=G.
Definition A series is normal if each Gi⊴Gi+1.
Definition The factors of a normal series are the quotients Gi+1/Gi.
Definition A series if proper if the containments are all strict Gi⊲Gi+1.
Definition A series for G is a refinement of another series for G, if it's just had zero or more new groups added in, a refinement of a normal series must be again normal. A refinement is proper if it's has at least one new group added.
Definition A composition series is a proper normal series which has no proper refinements. (so 1⊲G is a composition series for any nontrivial G).
Lemma Any proper normal series can be refined to a composition series.
proof: Just keep refining it until you have a composition series, a composition series must be finite since our groups are finite,.
Proposition A normal series 1=G0⊴G1⊴G2⊴…⊴Gn=G is a composition series iff each factor is simple.
proof: (⇐) No factor is trivial. If a factor wasn't simple, i.e. Gi/Gi+1 has a proper nontrivial normal subgroup B then B≃K/Gi for some Gi⊲K⊲Gi+1 (by Noether4) and this gives a proper refinement. (⇒) not done yet.
Definition Two series for G are equivalent if the factors of one are permutation of the factors of another.
Theorem Factors of a composition series are simple.
proof: Let us have a normal series with ⋯⊴Hi⊴Hi+1⊴⋯ where Hi+1/Hi isn't simple, say it has some normal subgroup Hi/Hi⊲N⊲Hi+1/Hi, then by (Noether4) lattice isomorphism we get the existence of a subgroup Hi⊲N′⊲Hi+1. A refinement of our series.
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