A univariate function f(x) is said to be even provided that f(x) = f(-x). Geometrically, such functions are symmetric about the y-axis. Examples of even functions include 1 (or, in general, any constant function), left bracketing bar x right bracketing bar , cos x, x^2, and e^(-x^2). An even function times an odd function is odd, while the sum or difference of two nonzero functions is even if and only if each summand function is even. The product or quotient of two even functions is again even.