Talk:Hartogs number

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[edit] Invalid (or at least unclear) "Proof"

The proof given appears to be invalid or at least so unclear as to be unconvincing. JRSpriggs 06:14, 26 June 2007 (UTC)

I'm working on it. You could have tagged it with {{expert}}.... — Arthur Rubin | (talk) 18:53, 1 December 2007 (UTC)

Can we assume that, in ZF, if X is a set, then X × X is a set. The proof I'm familiar with in is NBG, where that's one of the finite set of axioms encompassing comprehension. — Arthur Rubin | (talk) 19:03, 1 December 2007 (UTC)

See Kripke–Platek set theory#Proof that Cartesian products exist for a proof that Cartesian products exist. Or use the axiom of powerset#Consequences and the axiom schema of specification. JRSpriggs 22:11, 1 December 2007 (UTC)

And thanks for reworking the proof, it is much clearer now. JRSpriggs 22:27, 1 December 2007 (UTC)