Let G be a finite solvable group and F a formation.
Then
ResidualWrtFormation returns the F-residual subgroup
of G.
The following special cases have their own functions.
This is the last term of the descending central series of G.
This is the smallest normal subgroup of G whose index is
a power of the prime p.
This is the smallest normal subgroup of G whose index is
divisible only by primes in the list primes.
This is the smallest normal subgroup of G whose index is
divisible only by primes not in the list primes.
This is the smallest normal subgroup of G whose factor group
is a direct product of groups of prime order.