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205. Igor Klep, Markus Schweighofer:
Connes' embedding conjecture and sums of hermitian squares.

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Submission: 2006, July 24.

We show that Connes' embedding conjecture on von Neumann algebras is equivalent to the existence of certain algebraic certificates for a polynomial in noncommuting variables to satisfy the following nonnegativity condition: The trace is nonnegative whenever self-adjoint contraction matrices of the same size are substituted for the variables. These algebraic certificates involve sums of hermitian squares and commutators. We prove that they always exist for a similar nonnegativity condition where elements of separable II_1-factors are considered instead of matrices. Under the presence of Connes' conjecture, we derive degree bounds for the certificates.

There is an erratum available here.

Mathematics Subject Classification (2000): 11E25, 13J30, 58B34.

Keywords and Phrases: sum of squares, Connes' embedding conjecture, quadratic module, tracial state, von Neumann algebra.

Full text, 20p.: dvi 134k, ps.gz 222k, pdf 339k.

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