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Séminaire Bourbaki

Sylvy Anscombe — Shelah's Conjecture and Johnson's Theorem


The "Shelah Conjecture" proposes a description of fields whose first-order theories are without the Independence Property (IP): they are finite, separably closed, real closed, or admit a non-trivial henselian valuation. One of the most prominent dividing lines in the contemporary model-theoretic universe, IP holds in a theory if there is a formula that can define arbitrary subsets of arbitrarily large finite sets. In 2020, Johnson gave a proof of the conjecture in an important case; namely, the case of dp-finite (roughly: finite dimensional) theories of fields. Combined with a result of Halevi–Hasson–Jahnke, Johnson’s Theorem completely classifies the dp-finite theories of fields.

We will explain this classification, describe some ingredients of the proof, and explore how Johnson’s Theorem and the Shelah Conjecture fit into the bigger picture.