Invariants of Boundary Link Cobordism

Abstract

An n-dimensional \mu-component boundary link is a codimension 2 embedding of spheres L=\bigsqcup_{\mu}S^n \subset S^{n+2} such that there exist \mu disjoint oriented embedded (n+1)-manifolds which span the components of L. An F_\mu-link is a boundary link together with a cobordism class of such spanning manifolds. The F_\mu-link cobordism group C_n(F_\mu) is known to be trivial when n is even but not finitely generated when n is odd. Our main result is an algorithm to decide whether two odd-dimensional F_\mu-links represent the same cobordism class in C_{2q-1}(F_\mu) assuming q>1. We proceed to compute the isomorphism class of C_{2q-1}(F_\mu), generalizing Levine's computation of the knot cobordism group C_{2q-1}(F_1). Our starting point is the algebraic formulation of Levine, Ko and Mio who identify C_{2q-1}(F_\mu) with a surgery obstruction group, the Witt group G^{(-1)^q,\mu}(Z) of \mu-component Seifert matrices. We obtain a complete set of torsion-free invariants by passing from integer coefficients to complex coefficients and by applying the algebraic machinery of Quebbemann, Scharlau and Schulte. Signatures correspond to `algebraically integral' simple self-dual representations of a certain quiver (directed graph with loops). These representations, in turn, correspond to algebraic integers on an infinite disjoint union of real affine varieties. To distinguish torsion classes, we consider rational coefficients in place of complex coefficients, expressing G^{(-1)^q,\mu}(Q) as an infinite direct sum of Witt groups of finite-dimensional division Q-algebras with involution. Numerical invariants of such Witt groups are available in the literature.