Groups in which every subgroup has subnormal defect at most three.
Commutators, defect, subnormality, nilpotency, dedekindian groups.
In this work we study groups in which every subgroup has subnormal defect less than or equal to 2. We divide our investigation into the study of groups with defect 1 and 2. For groups with defect 1, called Dedekind groups, our main objective is to demonstrate the Dedekind-Baer Theorem that gives us a classification of non-abelian Dedekind groups. For groups with defect 2, we present the classes S, A and T and study the continence relations between them. Based in Mahdavianary and Heineken, we also show that groups in these classes are nilpotent with nilpotency class less than or equal to 3.