Computability of the Radon-Nikodym derivative

Abstract : We show that computability of the Radon-Nikodym derivative of a measure μ absolutely continuous w.r.t. some other measure λ can be reduced to a single application of the non-computable operator EC, which transforms enumeration of sets (in N) to their characteristic functions. We also give a condition on the two measures (in terms of the computability of the norm of a certain linear operator involving the two measures) which is sufficient to compute the derivative.