User:IssaRice/Little o notation

Can we write just $f\in o(g)$ or $f=o(g)$ or $f(x)\in o(g(x))$ or $f(x)=o(g(x))$? In general we can't because for this notation to make sense, we also need to know where the argument $x$ is going. In algorithms, we have $x\to \infty$, but in analysis (e.g. in some definitions of differentiability) we have $x\to 0$