Finite Permutation Groups with Few Orbits Under the Action on the Power Set

We study the orbits under the natural action of a permutation group $G \subseteq S_n$onthepowerset$\mathscr{P}({1, \dots , n})$. The permutation groups having exactly $n+1$ orbits on the powerset can be characterized as set-transitive groups and were fully classified in \cite{BP55}. In this paper, we establish a general method that allows one to classify the permutation groups with $n+r$ set-orbits for a given $r$, and apply it to integers $2 \leq r \leq 15$ using the computer algebra system GAP.