Dehn's lemma
In mathematics , Dehn's lemma asserts that a piecewise-linear map of a disk into a 3-manifold , with the map's singularity set in the disk's interior , implies the existence of another piecewise-linear map of the disk which is an embedding and is identical to the original on the boundary of the disk . 
<triplet> singularity <subj> mathematics <obj> part of