Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Let $X$ be a topological space and $f: X^2\to X$ be a projection onto the first factor.

Is $f$ continuous?

Thanks for your help.

share|cite|improve this question
up vote 12 down vote accepted

The continuity of $f$ strongly relies (of course) on the topology $X^{2}$ is provided with. If $X^{2}$ has the product topology, then the projections onto the factors are clearly continuous (if you like, this is just by definition of product topology, see here).

share|cite|improve this answer
So what is the answer? – hengxin Sep 25 '14 at 13:31
@hengxin Well, the answer is that, IF $X^2$ is endowed with the product topology (as it is usually understood to be), then $f$ is continuous. But, IF $X^2$ has an arbitrary topology, then the answer is that, in general, $f$ is not continuous. – Marco Vergura Sep 25 '14 at 14:05
Thanks. As a beginner of general topology, I am still often confused with so much definitions, and need some definite answers. – hengxin Sep 25 '14 at 14:09
@hengxin You are welcome! Anyway, as I more or less underlined above, if you find in a book sentences like: "Let $X$ and $Y$ be topological spaces and let $X\times Y$ be their product", then you can assume that $X\times Y$ is intended to denote the product of $X$ and $Y$ as topological spaces, that is the cartesian product of the underlying sets of $X$ and $Y$ endowed with the product topology. If this had not to be the case, the author would explicitly say so. – Marco Vergura Sep 25 '14 at 14:21
I have another question about the product space. Would you mind checking it out at your convenience? – hengxin Sep 26 '14 at 1:11

If $\{X_\omega\}_{\omega \in \Omega}$ is a family of topological spaces, it is standard practice to turn the corresponding Cartesian product $\prod_{\omega \in \Omega}X_\omega$ into a topological space as well by equipping it with the so-called "product topology."

Now, the important thing to realize is that this product topology is expressly defined so as to make all the projections $\pi_\alpha:\left( \prod_{\omega \in \Omega}X_\omega \right) \rightarrow X_\alpha$ continuous.1

Therefore, absent any specific information to the contrary, I'd say that the answer to your question is yes.

(But, as already mentioned, the question of continuity does depend critically on the topologies chosen—after all, it is the topologies of the domain and codomain that define which functions are continuous. This means that, if, contrary to standard practice, the topology assigned to your $X^2$ is not the standard product topology described above, then all bets are off.)

1And in fact, this product topology is defined to be the smallest (aka weakest) topology with this property. More specifically, the product topology is defined as the topology on $\prod_{\omega \in \Omega}X_\omega$ that is generated by the subbase

$$\bigcup_{\omega \in \Omega}\{\pi_\omega^{-1}(U):U\text{ is open in } X_\omega\}$$

This means that the product topology is the smallest topology on the product space that contains all the inverse images of open sets with respect to some projection $\pi_\omega$. Therefore, in this topology, the projections $\pi_\omega$ are rendered continuous by construction.

share|cite|improve this answer
Universal property for the win! – Sammy Black May 8 '13 at 9:20

If $U$ is open in $X$, then $U\times X$ is open in $X^2$.

share|cite|improve this answer
So what is the answer? – hengxin Sep 25 '14 at 13:31

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.