10

In any topological space $X$, the identity function on $X$ and any constant function $X\rightarrow X$ are necessarily continuous. Does there exist an infinite topological space such that these functions are the only continuous functions $X\rightarrow X$?

I require the space to be infinite since there are finite examples such as the one-point space, and the space $\{1,2\}$ with open sets $\{\emptyset,\{1\},\{1,2\}\}$. Although I am not aware of any finite examples with more than two points, so those could be of interest to me too.

The only further progress I've made is that such a space must be connected: otherwise write $X$ as a disjoint union of nonempty open sets $U\cup V$. A map sending $U$ to one point and $V$ to a different point is then continuous.

volcanrb
  • 3,114
  • A weaker condition is that the only homeomorphism is the identity. These spaces are called rigid. Spaces you are looking for are sometimes called strongly rigid. Searching for these notations you may find further information and examples. – Ulli Oct 28 '22 at 07:10
  • This question mentions an example of strongly rigid space. – PatrickR Nov 19 '22 at 09:30

2 Answers2

15

Yes, there exist such spaces, they can even be very nice! In 1967 Cook constructed a compact metric connected space, now known as the Cook continuum which has exactly the property you're asking for.

4

About your second question:
Let $X$ be a finite space with this property. Then $X$ has at most 2 elements:

Assume $|X| \ge 3$. $X$ is $T_0$ [Assume not, then there are distinct $x, y \in X$ such that $x \in \overline{\{y\}}$ and $y \in \overline{\{x\}}$. It is easy to see that the transposition $\tau_{x,y}$ which exchanges $x$ and $y$ and leaves every other element untouched, is a homeomorphism.]

Since $X$ is finite, there is a partial order $\le$ on $X$, such that the open sets are exactly the upper sets of $X$ with respect to $\le$.

There exist $a, b, c \in X$ such that $b < c$ and $b \nleq a$.
[If $X$ contains a chain of at least three elements, we are done. Since $X$ is not discrete, there exist $b, c \in X$ with $b < c$. If $b$ is not the minimum of $X$, we are done. So let $b=$ min $(X)$, all other elements incomparable. Pick $d \in X \setminus \{b, c\}$. Then it is easy to see that the transposition $\tau_{c,d}$ is a homeomorphism.This proves the claim.]

Now define
$f: X \rightarrow X, f(x) := \left\{ \begin{array}{c@{, }l} x, & b \le x \\ b, & b \nleq x \end{array} \right.$

It is easy to see that f is increasing, hence continuous. Moreover, $f(a) = b$, $f(c) = c \neq b$. Hence $f$ is neither the identity map, nor constant. Contradicition!

Jakobian
  • 15,280
Ulli
  • 6,241
  • 2
    When you say that the open sets are exactly the upper bounds of $X$ with respect to $\leq$, what do you mean? I would have thought that the upper bounds of $X$ are elements of $X$, not subsets of $X$ – Jon Sep 18 '24 at 08:43
  • 1
    @Jon: of course, it should read as "upper sets". Just corrected. Thank you very much! – Ulli Sep 19 '24 at 17:28
  • 1
    The order on $X$ is called specialization preorder, in this case a partial order, defined by $x\leq y$ iff $x\in\overline{{y}}$. – Jakobian Sep 19 '24 at 17:38