On Primeness of Labeled Oriented Trees
Knot complements are aspherical. Whether this extends to ribbon disc complements, or, equivalently, to standard 2-complexes of labeled oriented trees, remains unresolved. It is known that prime injective labeled oriented trees are diagrammatically reducible, that is, aspherical in a strong combinatorial sense. We show that arbitrary prime labeled oriented trees need not be DR. We conjecture that all injective labeled oriented trees are aspherical and prove the conjecture under natural conditions.
Harlander, Jens and Rosebrock, Stephan. (2012). "On Primeness of Labeled Oriented Trees". Journal of Knot Theory and Its Ramifications, 21(8), 1250077. http://dx.doi.org/10.1142/s0218216512500770