Example of transitive but not well ordered set needed

In summary, a set is ordinal if it is transitive and well-ordered by membership. However, there are examples of sets that are transitive but not well-ordered. One possibility is a set with an infinite descending chain, but this is not allowed in ZFC due to the axiom of regularity. Another example is { 0, { 0 }, { { 0 } } }, which is transitive but not well-ordered. This set does not contain any urelements and can be compared to an open interval on the real line or a set of nested Russian nesting dolls. Ultimately, while it may seem counter-intuitive, such sets are possible.
  • #1
dream runner
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Example of transitive but not well ordered set needed!

My question pertains to the definition of ordinals. According to Thomas Jech's edition of set theory, a set is ordinal if it is both transitive and well ordered by membership. I've been poking around trying to find an example of a set which is transitive and not well ordered by membership and only two possibilities seem to arise:

1: By definition, a set A is not well ordered by membership if there exists some subset B of A does not contain a least member. Thus every element b within B implies the existence of another element c within B which is also an element of b. This seems recursively to lead to an infinite set B.

2: A common counterexample I've seen is when proving that the class Ord of all ordinals is a proper class for else it would contain a member [alpha] which is an element of itself, and thus not well ordered. Furthermore, by the forum discussion on this, no set exists such that it is a member of itself and thus no set exists containing a set which is a member of itself, as this flies in the face of the Axiom Schema of Seperation.

It seems that both cases pose a stark contradiction and thus imply that no such set exists. However, absurdity is not a requisite for truth, thus I would like to know, since all evidence seems to point towards the contrary, whether it is even possible for any set A to be transitive by the definition that every element of A is also a subset of A, and yet not be well ordered by membership.


P.S. My idea of a set here does not include any urelements, which seems rational, for consider the set

A = {b, {b}}
The only way this set can possibly be transitive is if b is a subset of A, and the only non set element which inherently a member and a subset is the null element, thus
A = {null, {null}} is transitive, and adding another element would require that element be the successor {{null}}. Much like a russian nesting doll.
 
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  • #2


In the first example, I am not implying in any way that a transitive set cannot be infinite, it is the way in which it is infinite that seems counter-intuitive.

Let B be a subset of A having no least membered element.

Then if b is an element of B, then there exists c in B such that c is also in b, thus
b = {...,c}
but then c cannot be a least element, thus there must exist d in B such that d is in c
c = {...,d}
thus b = {...,{...,d}}
This continues on to create an infinite set B, but B cannot be inductive, or else null would be in B and by definition null is a least membered element. Thus B need be a non-inductive yet infinite set, containing some kind of infinite cascade of subset elements.

The situation seems comparable to an open interval of the real line. Imagine an infinite collection of Mitroshka dolls (the russian nesting dolls). There may exist a largest Mitroshka, but there are infinitely many smaller metroshka's within it. Much like an interval (a, b] of the real line which is also not well ordered in the regular sense of the word.

Is such a set possible?
 
  • #3


Not well-ordered by membership means that either two elements are not comparable or an infinite descending chain, the latter is forbidden by the axiom of regularity, assuming we're talking about ZFC. { 0, { 0 }, { { 0 } } } is an example of a transitive set that is not well-ordered.
 

FAQ: Example of transitive but not well ordered set needed

1. What is a transitive but not well ordered set?

A transitive but not well ordered set is a mathematical concept that refers to a set where the elements are related to each other in a transitive manner, meaning that if a is related to b and b is related to c, then a is also related to c. However, the set is not well ordered, meaning that it does not have a designated first element and every non-empty subset does not have a least element.

2. Can you provide an example of a transitive but not well ordered set?

One example of a transitive but not well ordered set is the set of all positive integers. This set is transitive because if a is a positive integer and b is also a positive integer, then the sum of a and b is also a positive integer. However, this set is not well ordered because there is no designated first element and every non-empty subset of positive integers does not have a least element.

3. How is a transitive but not well ordered set different from a partially ordered set?

A transitive but not well ordered set is different from a partially ordered set in that a partially ordered set must have a designated first element and every non-empty subset must have a least element. In a transitive but not well ordered set, there is no designated first element and every non-empty subset does not have a least element.

4. What are some real-life examples of transitive but not well ordered sets?

One real-life example of a transitive but not well ordered set is the set of all cities in a country. Cities can be related to each other in a transitive manner, such as if city A is connected to city B and city B is connected to city C, then city A is also connected to city C. However, there is no designated "first" city and there may not be a "least" city in every subset of cities (e.g. the largest city in a state may not be the "least" city in terms of population).

5. How are transitive but not well ordered sets used in mathematics and science?

Transitive but not well ordered sets are used in various fields of mathematics and science, such as in graph theory, topology, and order theory. They provide a useful tool for understanding relationships between elements in a set and can help in solving complex problems in these fields.

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