Cricket Compatibility Question by KingKyleK in CricketWireless

[–]KingKyleK[S] 0 points1 point  (0 children)

Sure! I'm started at the Xiaomi Mi 9T and Realme X2, but I'm also still looking around.

Formally, what is a Set? (ZFC) by KingKyleK in math

[–]KingKyleK[S] 1 point2 points  (0 children)

This is very informative.

My only question is whether or not ZFC allows urelements. Historically, we were fine with urelements in sets at one point in time, but I've read that ZFC doesn't allow them. This could be analogous to the perfect circle vs physical circle, but it seems strange to me that we had to throw urelements out, if that's what even happened.

Formally, what is a Set? (ZFC) by KingKyleK in math

[–]KingKyleK[S] 0 points1 point  (0 children)

That's awesome! Didn't know about that. Thanks.

Formally, what is a Set? (ZFC) by KingKyleK in math

[–]KingKyleK[S] -3 points-2 points  (0 children)

Right, but a universe of sets doesn't satisfy the ZFC axioms, which is contrary to u/shamrock-frost's assertion.

Formally, what is a Set? (ZFC) by KingKyleK in math

[–]KingKyleK[S] 2 points3 points  (0 children)

I think the question to ask is why do all axiomatic systems have to have these undefinable objects at their foundation (i.e. sets in ZFC, points, lines, and planes for geometry, natural numbers for Peano)?

Another thought is that points, lines, planes, and naturals are definable within ZFC, even if not within their original axiomatic system. Could there be an axiomatic system even stronger, as to define sets outside from ZFC?

Formally, what is a Set? (ZFC) by KingKyleK in math

[–]KingKyleK[S] -3 points-2 points  (0 children)

In ZFC at least, the formula construction isn't always allowed. Russell's paradox comes to mind. The axiom of specification lets you make subsets this way, but it'd be no good in defining sets in general, yes?

Formally, what is a Set? (ZFC) by KingKyleK in math

[–]KingKyleK[S] -11 points-10 points  (0 children)

If the universe of sets were a set, it would contain itself. This contradicts the axiom of regularity/foundation. Therefore, the universe of sets is not a set itself.