Big-O Estimate for Real Function/Examples/x is Big-O of x^2 at Infinity

Example of Big-$\OO$ Estimate for Real Function
Let $f: \R \to \R$ be the real function defined as:
 * $\forall x \in \R: \map f x = x$

Then:
 * $\map f x = \map \OO {x^2}$

Proof
Let us consider the real function $g: \R \to \R$ defined as:
 * $\forall x \in \R: \map g x = x^2$

Then we have that:
 * $\forall x \in \R_{>1}: \size {\map f x } < x^2$

Hence the result.