# Open but not continuous

In general there is no relation between the conditions of openness and continuity for functions between topological spaces.

For example, $f\colon (\mathbb{R}, \text{left ray topology})\to (\mathbb{R}, \text{discrete topology})$ given by $f(x)=x$ is open but not continuous.

My question if there is a function $(\mathbb{R}, \text{usual topology})\to (\mathbb{R}, \text{usual topology})$ which is open but not continuous?

And if so, what the conditions must be in this function to be open and not continuous?

• mathforum.org/library/drmath/view/62395.html – mfox Apr 22 '18 at 13:37
• Is there another example? – Tasneem Apr 22 '18 at 17:35
• Please use MathJax. – Alex Kruckman Apr 22 '18 at 19:06
• I edited the question for formatting and grammar. I left your last sentence as is, because I have no idea what it means. – Alex Kruckman Apr 22 '18 at 19:09

## 1 Answer

The Conway base 13 function $f: \mathbb{R} \to \mathbb{R}$ is a rather extreme example: it is nowhere continuous but $f[(a,b)] = \mathbb{R}$ for all $a < b$ in $\mathbb{R}$, and this trivially implies that $f$ is open.

• Beautiful example of the crazyness of real functions. – Surb Apr 22 '18 at 19:22