The cumulative hierarchy and the real numbers

In summary, the cumulative hierarchy is a method of constructing sets by starting with an empty set and building up sets from previous stages. Each level, denoted by $V_\alpha$, is the set of all sets formed from the previous levels. This method can be used to construct the real numbers by reaching a level with a set of size $|\mathbb{R}|$ and then using Dedekind cuts or Cauchy sequences. However, the sets in the hierarchy are not necessarily well-founded, which means that $\mathbb{R}$ may not be included in the hierarchy.
  • #1
hmmmmm
28
0
We define the cumulative hierarchy as:

$V_0=\emptyset$

$V_{\alpha+1}=\mathcal{P}(V_\alpha)$

If $\lambda$ is a limit ordinal then $V_\lambda=\bigcup_{\alpha<\lambda} V_\alpha$

Then we have a picture of a big V where we keep building sets up from previous ones and each $V_\alpha$ is the class (set?) of all sets formed from the previous stages.

Now I am wondering how we get from here to a construction of the real numbers? I can see that we will have a set of size $|\mathbb{R}|$ by $V_{\omega+2}$ and then we could go on to construct the reals formally via dedekind cuts of cauchy sequences. However are the sets in the hierarchy well founded in which case $\mathbb{R}$ would not be there?

Thanks for any help
 
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  • #2
I'm not too sure how to mark a thread as solved or something but my confusion here came from thinking that unions and power sets preserved well ordering, which they do not
 

FAQ: The cumulative hierarchy and the real numbers

What is the cumulative hierarchy?

The cumulative hierarchy is a mathematical concept that describes how sets are organized in a hierarchy based on their level of complexity. It starts with the empty set at the bottom and each subsequent level contains all the previous levels as subsets. This structure allows for the creation of infinite sets and helps to define the concept of infinity.

How is the cumulative hierarchy related to the real numbers?

The real numbers are a subset of the cumulative hierarchy. They are found at the highest level of the hierarchy and are considered to be the most complex type of set. The real numbers are used to represent continuous quantities such as length and time, and are essential for many mathematical calculations and theories.

What is the significance of the cumulative hierarchy in mathematics?

The cumulative hierarchy is an important concept in mathematics because it provides a foundation for understanding infinity and sets. It helps to define the concept of a limit and allows for the creation of infinite sets, which are essential in many areas of mathematics such as calculus and geometry.

Can the cumulative hierarchy be visualized?

Yes, the cumulative hierarchy can be visualized as a pyramid with the empty set at the bottom and each subsequent level containing all the previous levels as subsets. However, since the hierarchy is infinite, it is impossible to fully visualize it.

Are there any limitations to the cumulative hierarchy?

One limitation of the cumulative hierarchy is that it is based on the Zermelo-Fraenkel set theory, which has been proven to be incomplete. This means that there may be sets that exist outside of the hierarchy that cannot be defined using the rules of the set theory. Additionally, the hierarchy does not account for the existence of non-well-founded sets, which are sets that do not have a clear hierarchy structure.

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