On the Superstability Related with the Trigonometric Functional Equation

We will investigate the superstability of the (hyperbolic) trigonometric functional equation from the following functional equations: f(x + y) +/- g(x-y) = lambda f(x)g(y), f(x + y) +/- g(x-y) = lambda g(x)f(y), f(x + y) +/- g(x-y) = lambda f(x)f(y), f(x + y) +/- g(x-y) = lambda g(x)g(y), which can be considered the mixed functional equations of the sine function and cosine function, of the hyperbolic sine function and hyperbolic cosine function, and of the exponential functions, respectively. Copyright (C) 2009 Gwang Hui Kim.