Abstract
In the present note we give a new and short proof of Naimark's theorem asserting that for every commuting family ℱ of unitary operators in a πk-space Πk there exists ak-dimensional, nonpositive subspace invariant under ℱ.
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Sasvári, Z. New proof of Naimark's theorem on the existence of nonpositive invariant subspaces for commuting families of unitary operators in Pontryagin spaces. Monatshefte für Mathematik 109, 153–156 (1990). https://doi.org/10.1007/BF01302935
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DOI: https://doi.org/10.1007/BF01302935