# Positivity of a functional.

While going through the Riez Representation theorem i am stuck with the use of positivity of linear functional. My question is If $\tau$ is a linear functional from $C(X)\to \mathbb C$ , $f\in C(X)$ . Then i didn't understand how positivity is used to write the following :

$\tau(|f|^2) \le \tau (||f||^21) =||f||^2\tau(1)$

I want to know where exactly the positivity is used and how ?

Thank you for your kind help.

-

It is used in the inequality, because we have for all $x\in X$, $|f(x)|^2\leq \lVert f\rVert_{\infty}^2\mathbf 1(x)$, where $\mathbf 1$ is the function constant equal to $1$. Then we apply $\tau$ to $\lVert f\rVert_{\infty}^2\mathbf 1-|f|^2\geq 0$, and we conclude by linearity.