Given two functions $f$ and $g$ from $\mathbb{R}^+$ to $\mathbb{R}^+$ , the notation $$f=O(g)$$ means that the ratio $\displaystyle \frac{f(x)}{g(x)}$ stays bounded as $x\to\infty$ . If moreover that ratio approaches zero, we write $$f=o(g).$$

It is legitimate to write, say, $2x=O(x)=O(x^2)$ , with the understanding that we are using the equality sign in an unsymmetric (and informal) way, in that we do not have, for example, $O(x^2)=O(x)$ .

The notation $$f=\Omega(g)$$ means that the ratio $\displaystyle \frac{f(x)}{g(x)}$ is bounded away from zero as $x\to\infty$ , or equivalently $g=O(f)$ .

If both $f=O(g)$ and $f=\Omega(g)$ , we write $f=\Theta(g)$ .

One more notational convention in this group is $$f(x)\sim g(x),$$ meaning $\displaystyle \lim_{x\to\infty}\frac{f(x)}{g(x)}=1$ .

In analysis, such notation is useful in describing error estimates. For example, the Riemann hypothesis is equivalent to the conjecture $$\pi(x)=\operatorname{li} x+O(\sqrt{x}\log x),$$ where $\operatorname{li} x$ denotes the logarithmic integral.

Landau notation is also handy in applied mathematics, e.g. in describing the time complexity of an algorithm. It is common to say that an algorithm requires $O(x^3)$ steps, for example, without needing to specify exactly what is a step; for if $f=O(x^3)$ , then $f=O(Ax^3)$ for any positive constant $A$ .