# Math Help - Complex Analysis

1. ## Complex Analysis

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

2. Originally Posted by knguyen2005
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 $z_0$ in the set there is $\epsilon > 0$ such that $D(z_0,\epsilon)$ is contained in the set.
Use that definition.