Amenable L2-theoretic methods and knot concordance

We introduce new obstructions to topological knot concordance. These are obtained from amenable groups in Strebel's class, possibly with torsion, using a recently suggested $L^2$-theoretic method due to Orr and the author. Concerning $(h)$-solvable knots which are defined in terms of certain Whitney towers of height $h$ in bounding 4-manifolds, we use the obstructions to reveal new structure in the knot concordance group not detected by prior known invariants: for any $n>1$ there are $(n)$-solvable knots which are not $(n.5)$-solvable (and therefore not slice) but have vanishing Cochran-Orr-Teichner $L^2$-signature obstructions as well as Levine algebraic obstructions and Casson-Gordon invariants.