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mpitluk
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Why is there no number class containing all the ordinal numbers?
theorem4.5.9 said:That sounds right to me. I think we mean the same thing by number class and cardinal. A cardinal is an equivalence class of ordinals (hence your terminology). I am using some hand waving when I say pick an ordinal corresponding to the cardinal, but I'm not sure if there is a way to avoid choice.
theorem4.5.9 said:If there were such an ordinal, then it would be the largest ordinal. This is impossible, the easy answer being given any cardinal number (pick the one corresponding to the largest ordinal) you can create a larger cardinal by the powerset operation. This contradicts the assumption of a largest ordinal.
jgens said:That the class of all ordinals is a proper class has a simpler proof than this. If the class of all ordinals were not a proper class, then it would be an ordinal, which is a contradiction.
micromass said:A contradiction only if [itex]x\in x[/itex] is not allowed. If you allow that, then you need to look at x+1.
I didn't know that a cardinal number is an equivalence class of ordinals. I'm not entirely sure what this means. For example, you have some cardinal number, say, aleph_1 and then you have some set or class of all countable ordinals. Now, all I know is that the set of all countable ordinals makes up Cantor's second number class and has cardinality aleph_1. I'm not sure where "equivalence classes" comes in or how to use the term. Is an equivalence class a kind of number class? I guess I'm not to sure on what a number class is either, other than what different "kinds" of ordinals (finite, countable,...) are "members" of. But, I'm still unclear about what "equivalence classes" does here?theorem4.5.9 said:I think we mean the same thing by number class and cardinal. A cardinal is an equivalence class of ordinals (hence your terminology)
theorem4.5.9 said:As the name suggests, you use equivalence classes when you want, for all intensive purposes, to call two (technically) distinct objects the same.
Cardinality are the equivalence classes of ordinal numbers. So take the first infinite ordinal [itex]\omega[/itex]. Then [itex]\omega + 1[/itex] is also an ordinal. These two are in the same equivalence class, with the class as a whole being called the cardinal.
Well, it may be because, contrary to popular belief, regularity is not what prevents Russell's paradox from going through in ZFC. Indeed, if the other axioms allowed Russel's paradox to go through, then regularity would be powerless to stop it. Rather, it is the replacement of the axiom of unrestricted comprehension with the axioms of restricted comprehension, infinity, and replacement that allows the universe of sets to have a model. After that, the rest of the axioms (of ZF, not choice) just constitute conservative extensions, meaning they just narrow the class of models, not prevent there from being one.micromass said:I was just being annoying
For some reason I never see regularity as a proper axiom of ZFC. I don't know why I don't want to accept it...
Sweet, I didn't know that either.theorem4.5.9 said:...for all intents and purposes...
Nice. I didn't get it at all at first, as I had no idea what mod arithmetic was. But I got it (both, really) shortly after. So, if we are "working" mod 4, 2 + 3 = 1?theorem4.5.9 said:Take for example modular arithmetic. Let's assume we are working mod 12. Then we write such things as [itex]3 +4\equiv 7, 8+8\equiv 4[/itex]. It's as easy as clockwork (pun intended).
So, this equivalence is with respect to something right? They act as if they have the same cardinality, but certainly not as if they have the same order type. So, they are equivalence classes "with respect to" one-to-one correspondences, right? I think this is what jgens was saying...theorem4.5.9 said:Cardinality are the equivalence classes of ordinal numbers. So take the first infinite ordinal [itex]\omega[/itex]. Then [itex]\omega + 1[/itex] is also an ordinal. These two are in the same equivalence class, with the class as a whole being called the cardinal.
lugita15 said:... the rest of the axioms (of ZF, not choice) just constitute conservative extensions, meaning they just narrow the class of models, not prevent there from being one ...
Ordinal numbers are numbers used to indicate the position or order of something in a series, such as first, second, third, etc. They differ from cardinal numbers, which are used to count and represent quantity, such as one, two, three, etc.
Ordinal numbers are typically written with a letter or letters at the end, such as 1st, 2nd, 3rd, etc. When pronouncing them, the ending letters are usually omitted and the number is said as a cardinal number, for example, "first," "second," "third," etc.
Yes, ordinal numbers can be used in different languages. However, the way they are written and pronounced may vary from language to language. For example, in Spanish, "first" is written as "primero" and "second" as "segundo."
In mathematics, ordinal numbers are used to represent the order of elements in a set or sequence. They are also used in mathematical operations, such as addition and subtraction, to indicate the position of numbers in equations.
Yes, there are some exceptions and irregularities in the use of ordinal numbers. For example, "first" and "second" are often shortened to "1st" and "2nd," while "third" is written as "3rd" instead of "thrid." Additionally, some numbers have unique ordinal forms, such as "twelfth" and "twentieth."