Summary
A bounded, not necessarily everywhere defined, nonnegative operator A in a Hilbert space <InlineEquation ID=IE"1"><EquationSource Format="TEX"><![CDATA[<InlineEquation ID=IE"2"><EquationSource Format="TEX"><![CDATA[$]]></EquationSource></InlineEquation>]]></EquationSource></InlineEquation>\mathfrak{H}$ is assumed to intertwine in a certain sense two bounded everywhere defined operators B and C. If the range of A is provided with a natural inner product then the operators B and C induce two new operators on the completion space. This construction is used to show the existence of selfadjoint and nonnegative extensions of B*A and C*A.
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Hassi, S., Sebestyén, Z. & de Snoo, H. On the nonnegativity of operator products. Acta Math Hung 109, 1–14 (2005). https://doi.org/10.1007/s10474-005-0231-x
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DOI: https://doi.org/10.1007/s10474-005-0231-x