The Weak Atiyah Conjecture over a subfield of the complex numbers

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Let K\mathbb{K} be a subfield of C\mathbb{C}, and let GG be a countable group with a bound on the orders of its finite subgroups. For a finite matrix AA over K[G]\mathbb{K}[G], let rkN(G)(A)\mathrm{rk}_{\mathcal N(G)}(A) be its von Neumann rank. The Weak Atiyah Conjecture over K\mathbb{K}. There exists an integer n∈Zn\in\mathbb{Z} such that, for every finite matrix AA over K[G]\mathbb{K}[G],

rkN(G)(A)∈1nZ.\mathrm{rk}_{\mathcal N(G)}(A)\in\frac{1}{n}\mathbb{Z}.

This is a weakening of the Strong Atiyah Conjecture that retains the discreteness of the possible von Neumann ranks. The source describes it as wide open in general, although it is known for the groups established in the paper's theorem.

References

Primary source

Sam P. Fisher and Andrew Ng, “Outer automorphism groups and the Atiyah Conjecture”, arXiv:2606.19606 (2026).

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