On the zero-in-the-spectrum conjecture (Q5945545)

The authors answer a question of \textit{J. Lott} [Enseign. Math. (2) 42, 341-376 (1996; Zbl 0874.58086)] by showing: NEWLINENEWLINENEWLINETheorem 1: For any \(n\geq 6\), there exists a closed \(n\) dimensional smooth manifold \(M\) so that for any \(p=1,\dots ,n\), \(0\) does not belong to the spectrum of the Laplacian acting on the space of \(L^2\) forms on the universal covering of \(M\). NEWLINENEWLINENEWLINEThe authors prove Theorem 1 by restating it in an equivalent form using extended \(L^2\) homology: NEWLINENEWLINENEWLINETheorem 2: For any \(n\geq 6\), there exists a closed orientable smooth \(n\) dimensional manifold \(M\) so that the extended \(L^2\) homology \(H_p(M;\ell^2(\pi))\) vanishes for all \(p\) where \(\pi\) is the fundamental group of \(M\) and \(\ell^2(\pi)\) is the \(L^2\) completion of the group ring \(C[\pi]\). NEWLINENEWLINENEWLINEThis yields \(L^2\) invisible manifolds. Theorem 2 rests in turn on NEWLINENEWLINENEWLINETheorem 3: There exists a finite \(3\) dimensional polyhedron \(Y\) with fundamental group \(\pi=F\times F\times F\) where \(F\) is the free group with two generators such that the extended \(L^2\) homology \(H_p(Y;\ell^2(\pi))\) vanishes for all \(p=0,1,\dots \). NEWLINENEWLINENEWLINEThis yields as a corollary NEWLINENEWLINENEWLINETheorem 4: There exists an aspherical \(3\) dimensional finite polyhedron \(Z\) and a normal subgroup \(H\) in \(\pi=\pi_1(Z)\) such that the extended \(L^2\) homology \(H_p(Z;\ell^2(\pi/H))\) vanishes for all \(p=0,1,\dots\;\).