# nLab categoricity

Contepts

model theory

## Dimension, ranks, forking

• forking and dividing?

• Morley rank?

• Shelah 2-rank?

• Lascar U-rank?

• Vapnik–Chervonenkis dimension?

# Contepts

## Idea

In model theory, given a cardinal $\kappa$, a theory is $\kappa$-categorical (or categorical in cardinality $\kappa$), if it has precisely one isomorphism class of models of cardinality $\kappa$.

## Properties

The Morley categoricity theorem says that a first-order theory $T$ with countably many symbols is $\kappa$-categorical for one uncountable cardinal $\kappa$ iff $T$ is categorical in any uncountable cardinality.

There is much work beyond the first-order theories, for non-elementary classes, centered around (various versions of) Shelah’s Categoricity Conjecture especially in the setting of abstract elementary classes.

Study of categoricity lead historically to the development of the stability theory in model theory, see also geometric stability theory.

## References

• John Baldwin, Categoricity, Amer. Math. Soc. 2011, pdf

• John T. Baldwin, What is a complete theory, talk, pdf

• wikipedia: Morley categoricity theorem

• Saharon Shelah, Classification theory and the number of non-isomorphic models, 2nd ed. North Holland Amsterdam, 1990.

• Saharon Shelah, Categoricity of abstract classes with amalgamation, Annals of Pure and Applied Logic 98(1-3) 241–294, 1999.

• Boris Zilber, Uncountably categorical theories, Transl. Math. Monog., Amer. Math. Soc. 1993

• Rami Grossberg, Monica Vandieren, Shelah’s categoricity conjecture from a successor for tame AECs, pdf

• Michael Makkai, Saharon Shelah, Categoricity of theories $L_{\kappa\omega}$ with $\kappa$ a compact cardinal, Annals of Pure and Applied Logic 47, 41-97, 1990

• Christian Espíndola, An extension of Shelah’s eventual categoricity conjecture to accessible categories with directed colimits, arxiv/1906.09169; A short proof of Shelah’s eventual categoricity conjecture for AEC’s with interpolation, under GCH, arxiv/1909.13713; A topos-theoretic proof of Shelah’s eventual categoricity conjecture, Brno, April 30, 2020, video yt, slides pdf

Last revised on January 4, 2021 at 19:35:36. See the history of this page for a list of all contributions to it.