Recursive definability of curves and Jacobians in perfect fields

Let KK be a perfect field, let gg be a positive integer, and let XX and YY be definable sets. A genus-gg curve over KK with Jacobian JJ is represented by definable bijections between XX and C(K)C(K) and between YY and J(K)J(K).

Recursive definability conjecture. There are formulas ϕn,g(x)\phi_{n,g}(\vec{x}), ψn,g(x,y)\psi_{n,g}(\vec{x},\vec{y}), and ρn,g(x,z)\rho_{n,g}(\vec{x},\vec{z}), depending recursively on nn and gg, such that the following are equivalent:

  1. There is a genus-gg curve C/KC/K with Jacobian JJ for which XX is in definable bijection with C(K)C(K) and YY is in definable bijection with J(K)J(K).
  2. There are nNn\in\mathbb{N} and aϕn,g(K)\vec{a}\in\phi_{n,g}(K) such that XX is in definable bijection with ψn,g(a,K)\psi_{n,g}(\vec{a},K) and YY is in definable bijection with ρn,g(a,K)\rho_{n,g}(\vec{a},K).

This is the underlying definability assertion used in the proposed proof of recursive axiomatizability. The surrounding discussion presents it as an informal formulation of what must be made precise, rather than as a resolved result.

Sources & referencesView supporting material

Primary source

Will Johnson, “Counting mod n in pseudofinite fields”, arXiv:1912.07223 (2019).

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