Conjugacy classes of $\pi$-elements and nilpotent/abelian Hall $\pi$-subgroups
Let $G$ be a finite group and $\pi$ be a set of primes. We study finite groups with a large number of conjugacy classes of $\pi$-elements. In particular, we obtain precise lower bounds for this number in terms of the $\pi$-part of the order of $G$ to ensure the existence of a nilpotent or abelian Hall $\pi$-subgroup in $G$.