in that context. For example, the uniqueness part of the theorem has been used to classify certain classes of quantum channels. For the comparison of different channels and computation of their mutual fidelities and information another representation of the channels by their "Radon–Nikodym" derivatives introduced by Belavkin is useful. In the finite-dimensional case, Choi's theorem as the tracial variant of the Belavkin's Radon–Nikodym theorem for completely positive maps is also relevant. The operators from the expression are called the Kraus operators of Φ. The expression is sometimes called the operator sum representation of Φ. References M.-D. Choi, Completely Positive Linear Maps on Complex Matrices, Linear Algebra and its Applications, 10, 285–290 (1975). V. P. Belavkin, P. Staszewski, Radon–Nikodym Theorem for Completely Positive Maps, Reports on Mathematical Physics, v. 24, No 1, 49–55 (1986). V. Paulsen, Completely Bounded Maps and Operator Algebras, Cambridge University Press, 2003. W. F. Stinespring, Positive Functions on C*-algebras, Proceedings of the American Mathematical Society, 6, 211–216 (1955). Operator theory Operator algebras Theorems in functional analysis