Prove that the following sets are open:
1) The left half plane { z: Re z < 0}
2) The open disk D(z,r) for any z in the Complex set C and r > 0
Can someone give me hints how to do these questions? I have no idea
Thanks
What is the definition of open?
It means for any $\displaystyle z_0$ in the set there is $\displaystyle \epsilon > 0$ such that $\displaystyle D(z_0,\epsilon)$ is contained in the set.
Use that definition.