PL Recognition After Two $S^2\times S^2$ Stabilisations is PSPACE-Hard

Abstract

Let $Z$ be any fixed closed connected PL $4$-manifold. We give a polynomial-time many-to-one reduction from the compressed word problem in Thompson's group $F$ to fixed-target recognition of $Z{\#}_2(S^2\times S^2)$. Once a finite triangulation of $Z$ has been fixed, the construction sends a straight-line program $A$ to a closed triangulated PL $4$-manifold $X_{A,Z}$ such that $\mathrm{val}(A)=1$ in $F$ if and only if $X_{A,Z}$ is PL-homeomorphic to $Z{\#}_2(S^2\times S^2)$. Since the compressed word problem in $F$ is PSPACE-complete, every such recognition problem is PSPACE-hard. In particular, fixed-target recognition of ${\#}_n(S^2\times S^2)$ is PSPACE-hard for every fixed $n\ge 2$.

16 pages, comments welcome