Solvable normal subgroups of 2-knot groups

Abstract

If $X$ is an orientable, strongly minimal $PD_4$-complex and $\pi_1(X)$ has one end then it has no nontrivial locally-finite normal subgroup. Hence if $\pi$ is a 2-knot group then (a) if $\pi$ is virtually solvable then either $\pi$ has two ends or $\pi\cong\Phi$, with presentation $\langle{a,t}|ta=a^2t\rangle$, or $\pi$ is torsion-free and polycyclic of Hirsch length 4; (b) either $\pi$ has two ends, or $\pi$ has one end and the centre $\zeta\pi$ is torsion-free, or $\pi$ has infinitely many ends and $\zeta\pi$ is finite; and (c) the Hirsch-Plotkin radical $\sqrt\pi$ is nilpotent.