We study locally flat disks in $(\mathbb {C}P^2)^\circ :=({\mathbb {C}} P^2)\setminus \mathring {B}^4$ with boundary a fixed knot $K$ and whose complement has fundamental group $\mathbb {Z}$. We show that, up to topological isotopy relative to the (rel.) boundary, such disks necessarily arise by performing a positive crossing change on $K$ to an Alexander polynomial one knot and capping off with a $\mathbb {Z}$-disk in $D^4.$ Such a crossing change determines a loop in $S^3 \setminus K$ and we prove that the homology class of its lift to the infinite cyclic cover leads to a complete invariant of the disk. We prove that this determines a bijection between the set of rel. boundary topological isotopy classes of $\mathbb {Z}$-disks with boundary $K$ and a quotient of the set of unitary units of the ring $\mathbb {Z}[t^{\pm 1}]/(\Delta _K)$. Number-theoretic considerations allow us to deduce that a knot $K \subset S^3$ with quadratic Alexander polynomial bounds $0,1,2,4$, or infinitely many $\mathbb {Z}$-disks in $(\mathbb {C}P^2)^\circ$. This leads to the first examples of knots bounding infinitely many topologically distinct disks whose exteriors have the same fundamental group and equivariant intersection form. Finally, we give several examples where these disks are realized smoothly.

We show that the fundamental group of every enumeratively rationally connected closed symplectic manifold is finite. In other words, if a closed symplectic manifold has a non-zero Gromov–Witten invariant with two point insertions, then it has finite fundamental group. We also show that if the spherical homology class associated with such a non-zero Gromov–Witten invariant is holomorphically indecomposable, then the rational second homology of the symplectic manifold has rank one.

We perform a general study of the structure of locally compact modules over compactly generated abelian groups. We obtain a dévissage result for such modules of the form ‘compact-by-sheer-by-discrete’, and then study more specifically the sheer part. The main typical example of a sheer module is a polycontractible module, that is, a finite direct product of modules, each of which is contracted by some group element. We show that every sheer module has a ‘large’ polycontractible submodule, in some suitable sense. We apply this to the study of compactly generated metabelian groups. For instance, we prove that they always have a maximal compact normal subgroup, and we extend the Bieri–Strebel characterization of compactly presentable metabelian groups from the discrete case to this more general setting.