which is the entropy of P relative to Q. Continuing in this case, if μ μ is any measure on X for which densities p and q with P ( d x ) = p ( x ) μ μ ( d x ) and Q ( d x ) = q ( x ) μ μ ( d x ) exist (meaning that P and Q are both absolutely continuous with respect to μ μ ), then the relative entropy from Q to P is given as