Discrete confocal quadrics and checkerboard incircular nets
Jan Techter · DepositOnce · 2021
Confocal quadrics constitute a special example of orthogonal coordinate systems. In this cumulative thesis we propose two approaches to the discretization of confocal coordinates, and study the closely related checkerboard incircular nets. First, we propose a discretization based on factorizable solutions to an integrable discretization of the Euler-Poisson-Darboux equation. The constructed solutions are discrete Koenigs nets and feature a novel discrete orthogonality constraint defined on pairs of dual discrete nets, as well as a corresponding discrete isothermicity condition. The coordinate functions of these discrete confocal coordinates are explicitly given in terms of gamma functions. Secondly, we show that classical confocal coordinates and their reparametrizations along coordinate lines are characterized by orthogonality and the factorization property. We use these two properties to propose another discretization of confocal coordinates, while again employing the aforementioned discrete orthogonality constraint. In comparison to the first approach, this definition results in a broader class of nets capturing arbitrary reparametrizations also in the discrete case. We show that these discrete confocal coordinate systems may equivalently be constructed geometrically via polarity with respect to a sequence of classical confocal quadrics. Different sequences correspond to different discrete parametrizations. We give several explicit examples, including parametrizations in terms of Jacobi elliptic functions. A particular example of discrete confocal coordinates in the two-dimensional case is closely related to incircular nets, that is, congruences of straight lines in the plane with the combinatorics of the square grid such that each elementary quadrilateral admits an incircle. Thus, thirdly, we classify and integrate the class of checkerboard incircular nets, which constitute the Laguerre geometric generalization of incircular nets. Further aspects of the novel discrete orthogonality constraint are studied in the introduction of this thesis. These include discrete Lamé coefficients, discrete focal nets, discrete parallel nets, and discrete isothermicity, as well as the relation to pairs of circular and conical nets.