[′läg·ə‚rith·mik ‚dif·ə‚ren·chē′ā·shən] (mathematics) A technique often helpful in computing the derivatives of a differentiable function ƒ(x); set g (x) = log ƒ(x) where ƒ(x) ≠ 0, then g ′(x) = ƒ′(x)/ ƒ(x), and if there is some other way to find g ′(x), then one also finds ƒ′(x).