The recession cone of $\Omega$. If $d_u$ is a unit vector, show that $d_u\in R(\Omega)$ iff there exists unbounded sequence $\{ w^k \}_{k=1}^\infty\subset \Omega$ satisfying $$\frac{w^k}{\|w^k\|}\to d_u\quad as\;k\to\infty$$
I don't know where to start the proof. I've tried from the definition but failed. Maybe there needs some construction?
Please give me some hints. Any help would be appreciated.