## Simple Bratteli diagrams with a Gödel-incomplete C*-equivalence problem

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## Abstract:

An abstract simplicial complex is a finite family of subsets of a finite set, closed under subsets. Every abstract simplicial complex $\mathcal {C}$ naturally determines a Bratteli diagram and a stable AF-algebra $A(\mathcal {C})$. Consider the following problem: INPUT: a pair of abstract simplicial complexes $\mathcal {C}$ and $\mathcal {C}’$; QUESTION: is $A(\mathcal {C})$ isomorphic to $A(\mathcal {C}’)$? We show that this problem is Gödel incomplete, i.e., it is recursively enumerable but not decidable. This result is in sharp contrast with the recent decidability result by Bratteli, Jorgensen, Kim and Roush, for the isomorphism problem of stable AF-algebras arising from the iteration of the same positive integer matrix. For the proof we use a combinatorial variant of the De Concini-Procesi theorem for toric varieties, together with the Baker-Beynon duality theory for lattice-ordered abelian groups, Markov’s undecidability result, and Elliott’s classification theory for AF-algebras.## References

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## Additional Information

**Daniele Mundici**- Affiliation: Department of Mathematics “Ulisse Dini”, University of Florence, Viale Morgagni 67/A, 50134 Florence, Italy
- Email: mundici@math.unifi.it
- Received by editor(s): March 12, 2002
- Received by editor(s) in revised form: March 31, 2003
- Published electronically: June 24, 2003
- Additional Notes: Partially supported by MURST Project on Logic
- © Copyright 2003 American Mathematical Society
- Journal: Trans. Amer. Math. Soc.
**356**(2004), 1937-1955 - MSC (2000): Primary 46L35, 06F20, 20F10, 03D40, 52B20
- DOI: https://doi.org/10.1090/S0002-9947-03-03353-1
- MathSciNet review: 2031047