Notation for comparing the relative order of functions. We write f(x) = O(g(x)) if f(x)/g(x) remains bounded as x→∞, so, for example, (x + 1)(x + 2) = O(x²). The notation is also used in the neighbourhood of finite values; for example sinx = x + O(x³) denotes the fact that (sinx − x)/x³ remains bounded (in fact converges) as x→0. See also little o notation.