By K. Jänich and Markus Rost (8 pages)
Jänich, Klaus; Rost, Markus. Regularity of line defects in 3-dimensional media. Topology 24 (1985), no. 3, 353-60.
Related:
[Mapping defects in 4-manifolds]
by Markus Rost (140 pages)
Dissertation
Regensburger Mathematische Schriften 12, Universität Regensburg, Fachbereich Mathematik, Regensburg, 1987.
The thesis constructs for a topological space V its "defect complex DV". The homotopy classes of maps to DV are the homotopy classes of "regularly defect maps" to V. Then special emphasis lies on the computation of the third homotopy group π3(DV) for some specific V, for instance the Poincaré sphere.
The second homotopy group π2(DV) appeared as "regularly defect second homotopy group" in Regularity of line defects in 3-dimensional media.
The defect complex DV is described in both references here:
References: