This site is dedicated to open problems in nonstandard models of Paeno Arithmetic and related theory. As in most areas, we do not suffer from a shortage of open problems. The problems are arranged by category. Here is the list of categories.
3. Automorphisms and automorphism groups.
4. Standard systems and the Scott set problem.
5. Lattices of elementary substructures.
6. Uncountable models with interesting second-order properties.
7. Nonstandard satisfaction classes.
8. Model theory of strong fragments of arithmetic.
9. Model theory of weak fragments of arithmetic.
10. Model theoretic methods in proof theory.