Starting from the study of the orthoptic curves of parabolas and ellipses, we generalize to the case of isoptic curves for any angle, i.e. the geometric locus of points from which a parabola or an ellipse are viewed under a given angle. This leads to the investigation of spiric curves and to the construction of these curves as an actual intersection of a self-intersecting torus with a plane. The usage of a Computer Algebra System facilitated this investigation.
Dana-Picard, Thierry; Zehavi, Nurit; and Mann, Giora
"From conic intersections to toric intersections: The case of the isoptic curves of an ellipse,"
The Mathematics Enthusiast: Vol. 9
, Article 4.
Available at: http://scholarworks.umt.edu/tme/vol9/iss1/4