proof theoretic ordinal of zfc, and other formal systems

[lolgarithms]

what is the proof-theoretic strength (largest ordinal whose existence can be proved) for ZFC set theory?

[g_edgar]

If an ordinal \alpha can be proved to exist, so can \alpha+1

So in fact you probably want least ordinal whose existence cannot be proved

[lolgarithms]

If an ordinal \alpha can be proved to exist, so can \alpha+1

So in fact you probably want least ordinal whose existence cannot be proved

oops, my bad. what is it?

[lolgarithms]

could someone answer this question? please?
What is the least ordinal whose existence can't be proven in ZFC?

[lolgarithms]

what is the proof theoretic strenght of zfc? please help, i want to know!

plz, hurkyl, don't make me wait!!!

[g_edgar]

Why not research the question elsewhere on the Internet?

[Hurkyl]

plz, hurkyl, don't make me wait!!!
What? All I can do is make the obvious guess, and I'm not even sure that's well-defined, let alone the answer you seek.

[lolgarithms]

What? All I can do is make the obvious guess, and I'm not even sure that's well-defined, let alone the answer you seek.

if the smallest ordinal that can't be proven is well-defined: what determines that? that a stronger set theory is not known?

so we just call the ordinal "the proof theoretic ordinal of zfc"? ok. might not be an oridnal with a name, like kripke-platek ordinal

[Hurkyl]

if the smallest ordinal that can't be proven is well-defined: what is the guess?
The obvious guess would (IMHO) be something like the supremum of all of the ordinals in the constructible hierarchy.

However, I'm not sure the question is well-defined: why should "ZFC proves the existence of \beta" and "\alpha < \beta" should imply "ZFC proves the existence of \alpha".

[CRGreathouse]

Perhaps it isn't well defined. What is known, then, about the supremum of the set of definable ordinals in ZFC, and the infimum of undefinable ordinals in ZFC?

[Hurkyl]

Perhaps it isn't well defined. What is known, then, about the supremum of the set of definable ordinals in ZFC, and the infimum of undefinable ordinals in ZFC?
And that's where the extent of my knowledge ends. :smile: Although now that I think about it, the continuum hypotheses probably tells us some interesting information.