# Unsolvability of word problem for 2-knot complements

 Importance: High ✭✭✭
 Author(s): Gordon
 Subject: Topology
 Keywords: 2-knot Computational Complexity knot theory
\begin{problem} Does there exist a smooth/PL embedding of $S^2$ in $S^4$ such that the fundamental group of the complement has an unsolvable word problem? \end{problem}
It's known that there are smooth $4$-dimensional submanifolds of $S^4$ whose fundamental groups have unsolvable word problems. The complements of classical knots ($S^1 \to S^3$) are known to have solvable word problems, as do arbitrary $3$-manifold groups.