Is there a bounded connected set $X$ such that for all point $b$ there exists $r > 0$ such that $X \setminus O(b, r)$ is disconnect？

I find a question by myself, and I do not know if it is an interesting question.

Let $$X \subseteq \mathbb{R}^n$$ be a bounded connected set. And I define a "bad point" $$b \in \mathbb{R}^n$$ with respect to $$X$$ if there exists $$r > 0$$ such that $$X \setminus O(b, r)$$ is not connect, where $$O(b, r)$$ is a ball with center point $$b$$ and radius $$r$$. Let $$B(X)$$ be a set of all "bad point" with respect to $$X$$.

Strong question: Is there a bounded connected set $$X$$ such that $$B(X) = \mathbb{R}^n$$?

Weak question: Is there a bounded connected set $$X$$ such that $$X \subseteq B(X)$$?

I have no idea how to solve my question. I give some examples of my definition.

(1) If $$X$$ is a ball or spherical surface, then $$B(X) = \varnothing$$.

(2) If $$X = \{(x,0, \ldots, 0) \mid x \in [0,1] \}$$ is a close line segment, then $$B(X) = \{(x,a_{1}, \ldots, a_{n - 1}) \mid x \in (0,1), a_{i} \in \mathbb{R} \}$$

(3) If $$X = \{(x,0, \ldots, 0) \mid x \in (0,1) \}$$ is a open line segment, then $$B(X) = \{(x,a_{1}, \ldots, a_{n - 1}) \mid x \in (0,1), a_{i} \in \mathbb{R} \}$$

• Wouldn't you be able to take $X$ to be one entire axis? So $X = \{(x, 0, \ldots, 0) : x \in \mathbb R\}$, or do I misunderstand your question? This is essentially as your examples (2) and (3), but now $B(X) = \mathbb{R}^n$ and that should answer your strong question. – Mark Kamsma Apr 19 at 12:24
• @MarkKamsma Yes, I considered this example, I forget to write that X is bounded. – TeamBright Apr 19 at 12:34
• @YuiToCheng that is assuming we fix $r$ at the start, but the question is whether for every $b \in \mathbb R^n$ we can find $r$ such that $X - O(b, r)$ is disconnected. So $r$ can depend on $b$. – Mark Kamsma Apr 19 at 12:59
• @TeamBright I see, that makes the question less trivial. Still, you have already answered your own weak question with example (3). The strong question, I wouldn't know an answer to right away. – Mark Kamsma Apr 19 at 13:01
• @MarkKamsma Oh, yes. I answered the weak question. I did not realized it. I focused some sets which is $n$-dimensional set. – TeamBright Apr 19 at 13:18

As commented, the weak question is already solved.

For the strong one, I think there is a simple example if I've understood your question well. Consider a cross $$C$$ in $$\Bbb{R}^2$$ center at the origin, i.e the union of $$I_x=\{(x,0)\mid x\in[-1,1]\}$$ and $$I_y=\{(0,y)\mid y\in[-1,1]\}$$.

Any ball that covers $$(0,0)$$ by not the whole cross would disconnect $$C$$. But you can see that for any point $$b\in\Bbb{R}^2\setminus\{(0,0)\}$$, the (closed) ball $$O(b,|b|)$$ contains $$(0,0)$$ but not the whole cross (if you mean open ball then just take a slightly bigger radius). Note that for $$b=(0,0)$$ you can take a small radius and it will work as well.

So, for every $$b\in\Bbb{R}^2$$ there exists $$r>0$$ such that $$C\setminus O(b,r)$$ is disconnected. This example can be easily generalized to any higher $$\Bbb{R}^n$$.

• Love it, easy and elegant. Only case that is left is $n = 1$, but the answer to that is an (almost trivial) exercise left to the reader. – Mark Kamsma Apr 19 at 13:17
• Right, I didn't consider the case $n=1$, where that is surely not possible, since a connected set in $\Bbb{R}$ is an interval, so any point outside of the interval is not a bad point. @MarkKamsma – Javi Apr 19 at 13:20

Your weak question is most assuredly true -- just take $$X=\{(x,0,\ldots,0):x\in(-1,1)\}$$. Since deleting any sufficiently small neighborhood of any point in $$X$$ will leave behind two disjoint line segments, we are done.

Your strong questions is interesting, but I don't know how to answer it well, so bear with me as I describe a construction in $$\mathbb{R}^2$$:

Consider the curve (in polar coordinates) $$(r,\theta) = \left(\exp(f(\theta)),\theta\right)$$ where $$f(\theta)$$ is any continuous, monotonically increasing function with range $$\left(-\ln(2),0\right)$$, such as an appropriately scaled and translated $$\arctan$$ function. Take this curve, a circle with radius $$1/2$$ centered at the origin, and the unit circle, and define $$X$$ to be their union.

Where are the "bad points"? If you take a point outside the unit circle, say $$(0,2)$$, then we can find a radius $$r=1+\epsilon$$ which will "cut" the spiral off from the unit circle, leaving us a disconnected set. The same will be true for a point inside the circle of radius $$1/2$$, and a point inside the annulus can have a radius chosen to "cut" the two circular boundaries off from one another. Thus, all of $$\mathbb{R}^2$$ is "bad".

How do we take this construction and make it work in higher dimensions? There may or may not be an elegant equation for it, but the idea definetely generalizes -- take some increasingly fine "spiral" along the surface of hyper-sphere shells, and then take many such shells nested together, limiting towards to bounding shells, and link them together with line segments.

I wish I could describe that better, but I don't know how. Hopefully your mental picture is the same as mine.

EDIT: Please just use Javi's answer if you want a good example. This is far too complicated, even in $$\mathbb{R}^2$$.

• I do not directly see picture you are trying to describe, but isn't this unbounded? The $y$-coordinate ($\theta$) can get arbitrarily large. – Mark Kamsma Apr 19 at 13:12
• @MarkKamsma The polar coordinate description of the curve is unbounded in $\theta$, but the radius is bounded, leaving the entire object bounded. – ItsJustSamplingBro Apr 19 at 13:13
• Ah, I wasn't aware you were using polar coordinates. You may want to explicitly note that (even though you use the standard notation for it). On another note: once you have found an example in $\mathbb R^2$, I think you have an example for $\mathbb R^n$, $n \geq 2$, because you can just include your example as a subspace. – Mark Kamsma Apr 19 at 13:15
• @MarkKamsma Truthfully my example doesn't matter anymore, as Javi's answer has a much simpler construction that will work in higher dimensions. – ItsJustSamplingBro Apr 19 at 13:15
• Also thank you, you try to find a more general solution. It is a good approach. – TeamBright Apr 19 at 13:29