Finite groups, minimal bases and the intersection number

Let G $G$ be a finite group and recall that the Frattini subgroupFrat ( G ) ${\rm Frat}(G)$ is the intersection of all the maximal subgroups of G $G$ . In this paper, we investigate the intersection number of G $G$ , denotedα ( G ) $\alpha (G)$ , which is the minimal number of maximal subgroups whose intersection coincides withFrat ( G ) ${\rm Frat}(G)$ . In earlier work, we studiedα ( G ) $\alpha (G)$ in the special case where G $G$ is simple and here we extend the analysis to almost simple groups. In particular, we prove thatα ( G ) ⩽ 4 $\alpha (G) \leqslant 4$ for every almost simple group G $G$ , which is best possible. We also establish new results on the intersection number of arbitrary finite groups, obtaining upper bounds that are defined in terms of the chief factors of the group. Finally, for almost simple groups G $G$ we present best possible bounds on a related invariantβ ( G ) $\beta (G)$ , which we call the base number of G $G$ . In this setting,β ( G ) $\beta (G)$ is the minimal base size of G $G$ as we range over all faithful primitive actions of the group and we prove that the boundβ ( G ) ⩽ 4 $\beta (G) \leqslant 4$ is optimal. Along the way, we study bases for the primitive action of the symmetric groupS a b $S_{ab}$ on the set of partitions of[ 1 , a b ] $[1,ab]$ into a $a$ parts of size b $b$ , determining the exact base size fora ⩾ b $a \geqslant b$ . This extends earlier work of Benbenishty, Cohen and Niemeyer.