# Showing a topology is connected

How would I go about showing that $\mathbb{R}$ with the topology $\tau = \{ \emptyset \} \cup \{ U | \pi \in U \subseteq \mathbb{R}\}$ is connected?

Preferably an explanation someone who isn't good at topology could understand. Not sure where to start with this. I understand the being connected is a topological property and that something is connected if it;s not disconnected.

A topological space is disconnected if if there exist $U, V \in \tau$ which are nonempty and disjoint with X = U ∪ V

• Can you find two nonempty disjoint sets... at all? – G Tony Jacobs Dec 11 '17 at 15:15

So the idea is that you want to show that if you have two open sets $U$ and $V$ such that $U \cap V = \emptyset$ and $U \cup V = \mathbb{R}$, then one of either $U$ or $V$ must be the empty set.
Here's a big hint: in order for $U \cup V$ to be equal to $\mathbb{R}$, at least one of $U$ or $V$ must contain the element $\pi$ (suppose wlog that it's $U$). As $U$ and $V$ are open sets, hence in the set $\tau$, what does the property that $U \cap V = \emptyset$ mean the set $V$ must be?
• Technically the precise way I've set up the definition of connected means you can stop at the 'hence $V$ is empty' step, but yes that's correct. – Dan Rust Dec 11 '17 at 15:23
Hint: If the set $\mathbb{R}$ would not be connected in this topology, the definition tells you that then you could find two nonempty, open and disjoint sets $U_1$ and $U_2$ such that $\mathbb{R} = U_1 \cup U_2$.