A source transmits a symbol s, taken from {−4, 0, 4} with equal probability, over an additive white Gaussian noise channel. The received noisy symbol r is given by r = s + w where the noise w is zero mean with variance 4 and is independent of s. Using Q(x) = (1 / √(2π)) ∫_x^∞ e^−t2/2 dt, the optimum symbol error probability is