20D10 20D15 20D20 saturated formation 20F17 Finite group Fitting class Stone lattice \(\cal F\) -critical group,\(\cal F\) -maximal subgroup \(\cal F\)-hypercentre \(p\)-supersoluble groups boundary condition cover-avoidance property distributive lattice formation of finite groups hereditary formation local Fitting class local formation nilpotent group soluble group supersolublegroup the lattice of Fitting classes the lattice of formations