# Anti-orthotomics of frontals and their applications

## TL;DR

This paper introduces the concept of anti-orthotomics for frontals, establishes their properties, and explores applications including a generalized vector formula, optical interpretations, and criteria for frontals.

## Contribution

It defines the anti-orthotomic of a frontal relative to a point, proves its properties, and applies these results to geometric and optical problems.

## Key findings

- Anti-orthotomic of a frontal is uniquely defined and retains frontal properties.
- The anti-orthotomic satisfies specific geometric relations with the original frontal.
- Applications include a generalized Cahn-Hoffman vector formula, optical interpretations, and criteria for frontals.

## Abstract

Let $f: N^n\to \mathbb{R}^{n+1}$ be a frontal with its Gauss mapping $\nu: N\to S^n$ and let $P\in \mathbb{R}^{n+1}$ be a point such that $(f(x)-P)\cdot \nu(x) \ne 0$ for any $x\in N$. In this paper, for the mapping $\widetilde{f}: N\to \mathbb{R}^{n+1}$ defined by $$ \widetilde{f}(x)=f(x)-\frac{||f(x)-P||^2}{2(f(x)-P) \cdot \nu(x)}\nu(x), $$ the following four are shown. (1) $\widetilde{f}$ is a frontal with its Gauss mapping $\widetilde{\nu}(x)=\frac{f(x)-P}{||f(x)-P||}$ at $\widetilde{f}(x)$. (2) $\widetilde{f}$ is the unique anti-orthotomic of $f$ relative to $P$. (3) The property $(\widetilde{f}(x)-P)\cdot \widetilde{\nu}(x)\ne 0 $ holds for any $x\in N$. (4) The equality $||\widetilde{f}(x)-P||=||\widetilde{f}(x)-f(x)||$ holds for any $x\in N$. Moreover, three applications of the main result are given. As the first application, a generalization of Cahn-Hoffman vector formula is given. The second application is to clarify an optical meaning of anti-orthotomics. The third application gives a criterion to be a front for a given frontal.