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# Atiyah conjecture

In Mathematics, the **Atiyah conjecture** is a collective term for a number of statements about restrictions on possible values of <math>l^2</math>-Betti numbers.

## History

In 1976 Michael Atiyah introduced <math> l^2</math> -cohomology of manifolds with a free co-compact action of a discrete countable group (e.g. the universal cover of a compact manifold together with the action of the fundamental group by deck transformations.) Atiyah defined also <math>l^2</math>-Betti numbers as von Neumann dimensions of the resulting <math>l^2</math>-cohomology groups, and computed several examples, which all turned out to be rational numbers. He therefore asked if it is possible for <math>l^2</math>-Betti numbers to be irrational.

Since then, various researchers asked more refined questions about possible values of <math>l^2</math>-Betti numbers, all of which are customarily referred to as "Atiyah conjecture".

## Results

Many positive results were proven by Peter Linnell. For example, if the group acting is a free group, then the <math>l^2</math>-Betti numbers are integers.

The most general question open as of late 2011 is whether <math>l^2</math>-Betti numbers are rational if there is a bound on the orders of finite subgroups of the group which acts. In fact, precise relationship between possible denominators and the orders in question is conjectured; in the case of torsion-free groups this statement generalizes the zero-divisors conjecture. For a discussion see the article of B. Eckmann.

In the case there is no such bound, Tim Austin showed in 2009 that <math>l^2</math>-Betti numbers can assume transcendal values. Later it was shown that in that case they can be any non-negative real numbers.

## References

- Atiyah, M. F (1976). "Elliptic operators, discrete groups and von Neumann algebras".
*Colloque "Analyse et Topologie" en l'Honneur de Henri Cartan (Orsay, 1974)*. Paris: Soc. Math. France. pp. 43â€“72. Astérisque, No. 32â€“33.

- Austin, Tim (2009-09-12). "Rational group ring elements with kernels having irrational dimension". arXiv:0909.2360.

- Eckmann, Beno (2000). "Introduction to l_2-methods in topology: reduced l_2-homology, harmonic chains, l_2-Betti numbers".
*Israel J. Math.***117**. pp. 183â€“219.