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arXiv · 2607.23097

Reciprocal-Polar Linearization and Two Conic Constructions for the Cone Projection

George M. Georgiou
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Abstract

Let $$ f_R(x)=\frac{x}{1+\norm{x}/R},\qquad R>0, $$ be the radial cone projection of the plane onto the open disk $\DR=\{x:\norm{x}<R\}$. Previous work established the Self-Directrix and Confocal-Codirectrix Theorems, according to which $f_R$ maps focal conic arcs to focal conic arcs while preserving the distinguished focus and directrix. This note combines those results with three classical facts recorded by W.~H.~Besant. First, reciprocal polarity with respect to $\partial\DR$ represents every focal conic under consideration as the reciprocal polar of a unique circle, and in this circle representation the nonlinear action of $f_R$ becomes the elementary radius translation $q\mapsto q+R$. Second, the normals at the intersections of a fixed ray with the self-directrix family envelope an explicit parabola. Third, the tangent at $f_R(z)$ is constructed directly from the original point $z$ and the original line, without differentiating $f_R$ or solving for the conic.

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George M. Georgiou. 2026-07-25. Reciprocal-Polar Linearization and Two Conic Constructions for the Cone Projection. https://arxiv.org/abs/2607.23097

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