We study classes of groups whose subgroups of some infinite ranks are almost normal.Let ν be a certain property of subgroups. This property can be either internal (e.g., ν can be the property of being normal, subnormal, almost normal, permutable, or complementable) and external (in this case, ν is the property of being a subgroup that belongs to a certain class of groups X ). If G is a group, then Σ non-ν ( ) G (respectively, Σ ν ( ) G ) denotes the system of all subgroups G that do not possess the property ν (respectively, possess the property ν ). One the first problems in group theory (which still remains urgent) is the investigation of the influence of the systems Σ ν ( ) G and Σ non-ν ( ) G on the structure of a group for the most important natural properties ν. The classical paper [1] by Dedekind was the first step in this direction. In this paper, Dedekind studied finite groups all subgroups of which are normal, i.e., groups for which the system Σ norm ( ) G coincides with the system of all subgroups or the system Σ non-norm ( ) G is empty. Later, Miller and Moreno [2] studied finite groups all proper subgroups of which are Abelian, i.e., the system Σ ab ( ) G consists of all proper subgroups or Σ non-ab ( ) G = { G }. One should also mention Shmidt's paper [3], where he studied finite groups all proper subgroups of which are nilpotent, i.e., the system Σ nil ( ) G consists of all proper subgroups or Σ non-nil ( ) G = { G }. After these works, both finite and infinite groups for which the system Σ ν ( ) G is "sufficiently large" or the system Σ non-ν ( ) G is "sufficiently small" were thoroughly studied. This direction turned out to be quite interesting and fruitful, and numerous papers and several monographs have been devoted to it. In the present paper, we consider groups with restrictions imposed on the system Σ non-an ( ) G of all subgroups that are not almost normal.
A subgroup H of a group G is called almost normal in G if the set cl G H( the class of all subgroups conjugate to H ) is finite.If a subgroup H is normal in G, then cl G H ( ) = { H }, so that almost normal subgroups can be regarded as a natural generalization of normal subgroups. The subgroup H is almost normal in the group G if and only if its normalizer N H G ( ) has a finite index in G (this explains the name of these subgroups). In [4], Neumann characterized groups all subgroups of which are almost normal (i.e., the set Σ non-an ( ) G is empty) as groups with a center of finite index (groups finite over a center). In [5], Eremin generalized this result and proved that groups all Abelian subgroups of which are almost normal have a center of finite index. In [6], Eremin considered groups all infinite subgroups of which are almost normal (i.e., the set Σ non-an ( ) G consists of only finite subgroups). Locally almost solvable groups all infinite subgroups of which are almost normal were described by Semko, Levishchenko, and Kurdachenko in [7]. Later, the investigation of groups for which the system Σ non-an ( ) G is "sufficiently small"...