Given a solvable group, we can define its Fitting length or nilpotent length in the following equivalent ways:
- It is the minimum possible length of a Fitting series (also called a nilpotent series) for the group, where a Fitting series is a subnormal series where all the successive quotients are nilpotent groups.
- If the group is a finite solvable group: It is the length of the upper Fitting series of the group. The upper Fitting series is an ascending series defined via quotient-iteration of the Fitting subgroup, which is the join of all nilpotent normal subgroups. That is, we define as the trivial subgroup so that is the Fitting subgroup of .
- If the group is a finite solvable group: It is the length of the lower Fitting series of the group. The lower Fitting series is a descending series defined via iteration of the nilpotent residual, where the nilpotent residual is the intersection of all normal subgroups with nilpotent quotients. For finite groups, the nilpotent residual coincides with the hypocenter.
For an infinite group, a join of nilpotent normal subgroups need not be nilpotent and an intersection of nilpotent-quotient normal subgroups need not have nilpotent quotient. In both cases, the problem is that the nilpotency class may become unbounded. Although we can still define the Fitting subgroup and the nilpotent residual, the Fitting subgroup need not be nilpotent and the nilpotent residual need not have nilpotent quotient.