A real function $f: \R \to \R $ is linear (a linear function) if \[ f(\alpha x + \beta y) = \alpha f(x) + \beta f(y) \quad \text{for all } x, y, \alpha , \beta \in \R \]
\[ f(\alpha x + \beta y) = \alpha f(x) + \beta f(y) \quad \text{for all } x, y, \alpha , \beta \in \R \]