Wielandt subgroup

Origin
The notion of Wielandt subgroup was introduced by Wielandt in the paper Ober den Normalizator den Subnormalen Untergruppen, Math Z. 45 (1939), Pages 209-244.

Symbol-free definition
The Wielandt subgroup of a group is defined as the intersection of the normalizers of all its subnormal subgroups.

Definition with symbols
The Wielandt subgroup $$W(G)$$ of a group $$G$$ is defined as the intersection, over all $$H$$ subnormal in $$G$$, of the groups $$N_G(H)$$.

Smaller subgroup-defining functions

 * Baer norm: This is the intersection of normalizers of all subgroups
 * Center: This is contained inside the Baer norm as well

Fixed-point operator
A group equals its Wielandt subgroup if and only if every subnormal subgroup of the group is normal, or equivalently, if and only if the group is a T-group.