Join of subnormal subgroups

Symbol-free definition
A subgroup of a group is termed a join of subnormal subgroups if it can be expressed as the join of a collection of subnormal subgroups of the group.

Stronger properties

 * Weaker than::Normal subgroup
 * Weaker than::Subnormal subgroup
 * Weaker than::Join of finitely many subnormal subgroups

Weaker properties

 * Stronger than::Subnormal automorphism-invariant subgroup