Published online by Cambridge University Press: 20 November 2018
We give a topological interpretation of the core group invariant of a surface embedded in ${{S}^{4}}\,\left[ \text{F-R} \right],\,\left[ \text{Ro} \right]$. We show that the group is isomorphic to the free product of the fundamental group of the double branch cover of ${{S}^{4}}$ with the surface as a branched set, and the infinite cyclic group. We present a generalization for unoriented surfaces, for other cyclic branched covers, and other codimension two embeddings of manifolds in spheres.