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arXiv · 2607.25064

Elementary equivalence of convex bodies in affine and projective languages

David Victor Feldman
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Abstract

Convex subsets of R^n carry two first-order structures: barycentric (affine) structure, with operations C_l(p,q)=(1-l)p+lq for l in [0,1], and betweenness (projective) structure, with ternary relation B(a,x,b) meaning x lies in [a,b]. Isomorphism means affine equivalence in the first and, for n at least 2 and sets open or closed, projective equivalence in the second. We ask when elementary equivalence already determines the body. Main theorem: two compact convex bodies of any dimension, with no regularity hypotheses, are elementarily equivalent in the barycentric language if and only if they are affinely equivalent. The proof rests on a definable compact family of gauges: simplices stationary for barycentric coordinates, with volume bounded below via the anticomplementary simplex. For the betweenness language we develop an interior von Staudt calculus, all quantifiers ranging over the body, making harmonic conjugacy and rational cross-ratio comparisons first-order; a relativization scheme then propagates planar separations to R^n. Consequences: the closed unit ball is separated from sum x_i^4 <= 1 for every n at least 2; and in the plane, projective categoricity holds outright for convex polygons and for bodies with real-analytic, positively curved, non-conic boundary, the latter via a definable finite projective invariant, the conic-cluster set: the points whose every boundary arc contains six co-conic extreme points. A general reduction isolates what remains of the projective conjecture: recovery of boundary coordinates in dimension at least three, and a definable compact gauge, obstructed exactly by non-compact projective symmetry, as on the quadric. For closed noncompact bodies the asymptotic structure is itself elementary: the dimensions of the recession cone and of the lineality space are determined by the betweenness theory, separating the solid cylinder from the slab.

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David Victor Feldman. 2026-07-27. Elementary equivalence of convex bodies in affine and projective languages. https://arxiv.org/abs/2607.25064

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