Isoptics of a closed strictly convex curve

Integral formula for secantoptics and its application

“…We derive the differential equation for this function and, using the formula (2.1), we estimate the value of its first derivative at the left end of the domain. This reasoning is a generalization of results obtained for isoptics in [3]. …”

On a question of T. Sheil-Small regarding valency of harmonic maps

“…Let C α be an α-isoptic of C ∈ C. We recall that an α-isoptic C α of C consists of those points in the plane from which the curve is seen under the fixed angle π − α, see [2], [3]. C α has the form…”

“…Moreover, we have (4.7) ∂λ ∂α = −µ sin α > 0, see [3]. We consider a family of all annuli CC α and the function Hence and from the definition of c o (α) it follows immediately that the mapping α → c o (α) is strictly increasing.…”

Isoptic surfaces of polyhedra

“…In [2] and [3] the isoptic curves of the closed, strictly convex curves are studied, using their support function. The papers [21] and [22] deal with curves having a circle or an ellipse for an isoptic curve.…”