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This paper has been withdrawn by the authors. The authors have now written two separate papers, one theoretical (A. Smerzi, A. Trombettoni, P. G. Kevrekidis, and A. R. Bishop Phys. Rev. Lett. 89, 170402 (2002)) and one experimental (F. S.…

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Temporarly withdrawn. Larger version of this review is currently in preparation.

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This paper has been withdrawn by the authors due to some errors.

This paper has been withdrawn by the author due to a crucial error in the formulation.

The paper was withdrawn by the authors.

This paper has been withdrawn by the authors. We have discovered an error in the evaluation of the diagram, which invalidates our conclusion.

This paper has been withdrawn by the authors, as it has been rejected in November 2007.

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This paper has been withdrawn by the author. The conjecture follows from the finiteness of group von Neumann algebras.

The paper was withdrawn by the author. It contained various errors.

Withdrawn by the authors due to submitting policy required by some journals.

This paper has been withdrawn by the authors; its contents are superseded by that of hep-th/0105305 and hep-th/0212275.