Projective categoricity conjecture for compact convex bodies

From papers

Let K,KK,K' be compact convex bodies in Rn\mathbb{R}^n, with n2n\ge2. Write KLBKK\equiv_{\mathcal{L}_{\mathrm{B}}}K' when they satisfy the same sentences in the projective betweenness language, and call them projectively equivalent when a projective transformation maps one onto the other.

Projective categoricity conjecture. KLBKK\equiv_{\mathcal{L}_{\mathrm{B}}}K' if and only if KK and KK' are projectively equivalent.

This is the projective analogue of the proved affine categoricity theorem. The paper explains that the affine reconstruction transposes only partly to the projective setting; the conjecture remains open.

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Sources & referencesView supporting material

Primary source

David Victor Feldman, “Elementary equivalence of convex bodies in affine and projective languages”, arXiv:2607.25064 (2026).

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