Shadows of a Closed Curve
Michael Gene Dobbins, Heuna Kim, Luis Pedro Montejano, Edgardo Roldán-Pensado · International Mathematics Research Notices · 2018
Abstract A shadow of a geometric object A in a given direction v is the orthogonal projection of A on the hyperplane orthogonal to v. We show that any topological embedding of a circle into Euclidean d-space can have at most two shadows that are simple paths in linearly independent directions. The proof is topological and uses an analog of basic properties of degree of maps on a circle to relations on a circle. This extends a previous result that dealt with the case d = 3.