All two dimensional links are null homotopic
arXiv:math/0004021 · doi:10.2140/gt.1999.3.235
Abstract
We show that any number of disjointly embedded 2-spheres in 4-space can be pulled apart by a link homotopy, ie, by a motion in which the 2-spheres stay disjoint but are allowed to self-intersect.
18 pages. Published copy, also available at http://www.maths.warwick.ac.uk/gt/GTVol3/paper10.abs.html