¹The error function erf is defined as erf(z) = ∫ ₀^zexp(-t²)dt. Sometimes the complementary error function is used, erfc=1-erf. Alternatively the Q-function can be used, Q(z) = ∫ _z^∞exp(-t²∕2)dt, which is related to the complementary error function by Q(x) = erfc.