Polynormal subgroup

A **polynormal subgroup** is a concept from group theory, particularly in the study of group extensions and solvable groups. A subgroup \( N \) of a group \( G \) is called **polynormal** if for every finite sequence of subgroups \( H_1, H_2, \ldots, H_n \) of \( G \) such that: 1. \( H_1 \) is a subgroup of \( N \), 2.