Note 1 to entry: The value of the function f of n variables $x_1,x_2,\dots ,x_n$ is denoted by $f(x_1,x_2,\dots ,x_n)$.

Note 2 to entry: In many practical applications in electrotechnology, functions of one variable are used. This variable is often the duration and the function is then called "function of time" and denoted by f(t). But the variables may also be space variables (e.g. coordinates), frequency, etc.