We solve two problems in the theory of correspondences that have important
implications in the theory of product systems. The first problem is the question
whether every correspondence is the correspondence associated (by the representation
theory) with a unital endomorphism of the algebra of all adjointable operators on a
Hilbert module. The second problem is the question whether every correspondence
allows for a nondegenerate faithful representation on a Hilbert space. We also solve
an extension problem for representations of correspondences and we provide new
efficient proofs of several well-known statements in the theory of representations of
W$^∗$–algebras.
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