# Definition of Connectedness .... What's wrong with my informal thinking ....?

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In summary: Your Name]In summary, Duistermaat and Kolk's definition of disconnectedness/connectedness states that for a set to be disconnected, it must be the disjoint union of two open sets in the Euclidean topology. This means that the boundary points may not necessarily be contained within the sets themselves, but they are still considered open in the Euclidean topology. Therefore, in the equation (A ∩ U) ∪ (A ∩ V) = A, the boundary points are not included in the intersection, but this does not affect the result.
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I am reading "Multidimensional Real Analysis I: Differentiation" by J. J. Duistermaat and J. A. C. Kolk ...

I am focused on Chapter 1: Continuity ... ...

I need help with an aspect of D&K's definition of disconnectedness/connectedness ... ...

Duistermaat and Kolk's definition of disconnectedness/connectedness reads as follows:https://www.physicsforums.com/attachments/7774I tried to imagine the typical case of a disconnected set $$\displaystyle A$$ ... there would be two open sets $$\displaystyle U, V \text{ in } \mathbb{R}^n$$ ... indeed both sets could be contained in $$\displaystyle A$$ and we would require, among other things that

$$\displaystyle ( A \cap U) \cup (A \cap V) = A$$

This, I imagine, would give a situation like the figure below:
View attachment 7775
BUT ... since $$\displaystyle U, V$$ are open they cannot contain their boundary ... but if they do not contain their boundary ... I am assuming a common boundary ... how can we ever have $$\displaystyle ( A \cap U) \cup (A \cap V) = A$$ ...Can someone explain where my informal thinking is going wrong ... since there obviously must exist disconnected sets ...Help will be appreciated ...

Peter

Hi Peter,

There is no reason why the boundary would be in $A$; that is the whole point of disconnected sets.

As an example, take $A=\mathbb{R}\setminus\{0\}$. This is the disjoint union of two open sets.

Hi Peter,

I understand your confusion. The key here is to remember that when we say "open sets U, V in R^n," we mean open sets in the Euclidean topology. In this topology, the boundary of an open set is not necessarily contained within the set itself. So, in the figure you provided, the boundary of U and V may not be contained within the sets, but they are still open sets in the Euclidean topology.

Therefore, when we say (A ∩ U) ∪ (A ∩ V) = A, we are not including the boundary points in the intersection. This is because the boundary points are not necessarily contained within the sets themselves. So, in your example, the boundary points would not be included in (A ∩ U) ∪ (A ∩ V), but this does not affect the overall result of the equation.

I hope this helps clarify the concept for you. Let me know if you have any other questions.

## 1. What is the definition of connectedness?

The definition of connectedness refers to the state of being connected or joined together, either physically, emotionally, or in terms of ideas or concepts.

## 2. How does connectedness impact our lives?

Connectedness can have a significant impact on our lives as it can affect our relationships, sense of belonging, and overall well-being. It can also shape our perspectives and influence our decisions and behaviors.

## 3. Why is connectedness important in scientific research?

In scientific research, connectedness is crucial as it helps to establish relationships between different variables and concepts. It allows scientists to see how different factors interact and impact each other, leading to a better understanding of the subject being studied.

## 4. What's the difference between connectedness and correlation?

Connectedness and correlation are often used interchangeably, but they are not the same. Connectedness refers to a relationship between two or more things, while correlation specifically refers to a statistical measure of the strength and direction of the relationship.

## 5. What's wrong with using informal thinking when discussing connectedness?

Informal thinking can lead to biased or inaccurate conclusions about connectedness as it relies on personal opinions and experiences rather than evidence and scientific research. It may also overlook important factors and connections, leading to a limited understanding of the concept.

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