Talk:Class is Extensional

It occurred to me that with the new name for this result: Class is Extensional, we need to include a link to the term "extensional".

Turns out we have no such definition yet. Anyone care to do the appropriate research to set this up, with its usual $\mathsf{Pr} \infty \mathsf{fWiki}$ rigour? --prime mover (talk) 08:19, 17 March 2022 (UTC)

For now I have linked to Axiom:Axiom of Extension (Classes) as that is what it is. Any further coverage should be done in that realm, indicating that Takeuti/Zaring apparently use a different axiomatisation. As I consider T/Z more a historical source than a useful reference for today's mathematics, I won't be picking that task up any time soon. But at least now the link is up and the terminology is explained. — Lord_Farin (talk) 08:27, 18 March 2022 (UTC)

I spent some time working my way through the Smullyan and Fitting work a couple of years back, and later the Just and Weese work, but took a break to do other things instead, and never got back to them. I fully expect that a rigorous completion of both of these should give a basic coverage of all this basic set and class theory material, as they are as contemporary as can be, explain everything well in lucid language, and have masses of examples and exercises. But it's hard work and I have promised myself I would break the spirit of the exercises of the initial chapters of the formidable Seth Warner work first. --prime mover (talk) 08:56, 18 March 2022 (UTC)