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In the algebraic approach, the fundamental structure of a quantum theory is given by an abstract algebra of the canonical commutation relations, which can be given various different representations in terms of subalgebras of the bounded operators on a Hilbert space. Two such representations, each consisting of a Hilbert space H and the set of bounded operators ˆO defined on it, are unitarily equivalent if there is a (one-to-one, linear, norm preserving) map U:G→H′ such that (∀i)(U−1ˆO′iU=ˆOi). The Stone–von Neumann theorem guarantees that in the finite-dimensional case all irreducible representations of the abstract algebra are unitarily equivalent, but in the infinite-dimensional case there are unitarily inequivalent representations of the algebra.