Keywords: normal subgroup; abnormal subgroup; minimal non-$\mathcal {C}$-group
Summary: A group $G$ is said to be a $\mathcal {C}$-group if for every divisor $d$ of the order of $G$, there exists a subgroup $H$ of $G$ of order $d$ such that $H$ is normal or abnormal in $G$. We give a complete classification of those groups which are not $\mathcal {C}$-groups but all of whose proper subgroups are $\mathcal {C}$-groups.
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