20D10 20D20 20D15 saturated formation 20E28 20F17 Finite group Fitting class Stone lattice Sylow subgroup \(\cal F\) -critical group,\(\cal F\) -maximal subgroup \(\cal F\)-hypercentre \(c\)-normal subgroup \(p\)-supersoluble groups \(s\)-permutable group boundary condition cover-avoidance property distributive lattice finite group formation of finite groups hereditary formation local Fitting class local formation maximal subgroup nilpotent group quasinormal group soluble group supersoluble group supersolublegroup the lattice of Fitting classes