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On the algebraic structure of the unitary group

Abstract

We consider the unitary group \U of complex, separable, infinite-dimensional Hilbert space as a discrete group. It is proved that, whenever \U acts by isometries on a metric space, every orbit is bounded. Equivalently, \U is not the union of a countable chain of proper subgroups, and whenever \E\subseteq \U generates \U, it does so by words of a fixed finite length

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