Knot concordance and von Neumann $\rho$-invariants
  We prove the nontriviality, at all integral levels n, of the filtration, F_n, of the classical topological knot concordance group recently defined by the authors and Kent Orr [COT]. Recall that this filtration is significant not only because of it's strong connection to Whitney tower constructions of Casson and Freedman, but also because all previously-known concordance invariants are related to the first few terms in the filtration. In [COT] we proved nontriviality at the first new level (n=3) by using von Neumann $\rho$-invariants of the 3-manifolds obtained by zero surgery on the knots. Here, for larger n, we use the Cheeger-Gromov estimate for such $\rho$-invariants, as well as some rather involved algebraic arguments using our noncommutative Blanchfield forms. In addition, we consider a closely related filtration, defined in terms of Gropes in the 4-ball and show that this filtration is non-trivial for all n>2. 
Tim D. Cochran (Rice University), Peter Teichner (University of   California at Berkeley)