where P ( d x ) Q ( d x ) is the Radon–Nikodym derivative of P with respect to Q, i.e. the unique Q almost everywhere defined function r on X such that P ( d x ) = r ( x ) Q ( d x ) which exists because P is absolutely continuous with respect to Q. Also we assume the expression on the right-hand side exists. Equivalently (by the chain rule), this can be written as