This paper computes Whitney tower filtrations of classical links. Whitney
towers consist of iterated stages of Whitney disks and allow a tree-valued
intersection theory, showing that the associated graded quotients of the
filtration are finitely generated abelian groups. Twisted Whitney towers are
studied and a new quadratic refinement of the intersection theory is
introduced, measuring Whitney disk framing obstructions.
The first part of this paper completes the classification of Whitney towers
in the 4-ball that was started in three related papers. We provide an algebraic
framework allowing the computations of the graded groups associated to
geometric filtrations of classical link concordance by order $n$ (twisted)
Whitney towers in the 4-ball. Higher-order Sato-Levine invariants and
higher-order Arf invariants are defined and shown to be the obstructions to
framing a twisted Whitney tower.
This paper describes grope and Whitney tower filtrations on the set of
concordance classes of classical links in terms of class and order
respectively. Using the tree-valued intersection theory of Whitney towers, the
associated graded quotients are shown to be finitely generated abelian groups
under a (surprisingly) well-defined connected sum operation. Twisted Whitney
towers are also introduced, along with a corresponding quadratic enhancement of
the intersection theory for framed Whitney towers that measures Whitney-disk
framing obstructions.