Shape (In)dependent Inequalities for Triangleland's Jacobi and Democratic-Linear Ellipticity Quantitities

Abstract

Sides and medians are both Jacobi coordinate magnitudes, moreover then equably entering the spherical coordinates on Kendall's shape sphere and the Hopf coordinates. This motivates treating medians on the same footing as sides in triangle geometry and the resulting Shape Theory. In this paper, we consequently reformulate inequalities for the medians in terms of shape quantities, and proceed to find inequalities on the mass-weighted Jacobi coordinates. This work moreover identifies the $4/3$ -- powers of which occur frequently in the theory of medians -- as the ratio of Jacobi masses. One of the Hopf coordinates is tetra-area. Another is anisoscelesness, which parametrizes whether triangles are left-or-right leaning as bounded by isoscelesness itself. The third is ellipticity, which parametrizes tallness-or-flatness of triangles as bounded by regular triangles. Whereas tetra-area is clearly cluster choice invariant, Jacobi coordinates, anisoscelesness and ellipticity are cluster choice dependent but can be `democratized' by averaging over all clusters. Democratized ellipticity moreover trivializes, due to ellipticity being the difference of base-side and median second moments, whose averages are equal to each other. Thus we introduce a distinct `linear ellipticity' quantifier of tallness-or-flatness of triangles whose democratization is nontrivial, and find inequalities bounding this. Some of this paper's inequalities are shape-independent bounds, whereas others' bounds depend on the isoperimetric ratio and arithmetic-to-geometric side mean ratio shape variables.

arXiv comment22 pages including 8 figures

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• $N$-Body Problem: Minimal $N$'s for Qualitative Nontrivialities
• Monopoles of Twelve Types in 3-Body Problems
• Topological Shape Theory
• Background Independence: ${\mathbb{S}}^1$ and $\mathbb{R}$ absolute spaces differ greatly in Shape-and-Scale Theory
• Rubber Relationalism: Smallest Graph-Theoretically Nontrivial Leibniz Spaces
• Quadrilaterals in Shape Theory. II. Alternative Derivations of Shape Space: Successes and Limitations