Lecture Notes on Bayesian Estimation and Classification
《Lecture Notes on Bayesian Estimation and Classification》 · 第 143 页 · Section 5.3「Groups of Prime-Power Order」，小节「Groups in Which Every Subgroup Is Normal」
In $Q=Q_8$, the quaternion group of order $8$, there is only one element of order $2$ and it generates $Q'$. Hence $1\ne H\leq Q$ implies that $Q'\leq H$ and $H\triangleleft Q$. So every subgroup of $Q$ is normal. Our aim is to classify all groups with this property: these are known as Dedekind groups (a nonabelian Dedekind group is called Hamiltonian).