Timeline for Unions of sets exist? [closed]
Current License: CC BY-SA 2.5
12 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Apr 16, 2018 at 10:31 | comment | added | Mikhail Katz | I voted to reopen this question even though it is not exactly at the MO level, because it generated some interesting answers. | |
| Apr 16, 2018 at 1:38 | review | Reopen votes | |||
| Apr 16, 2018 at 22:46 | |||||
| Nov 3, 2010 at 18:19 | comment | added | Sasha | Thank you all that answered, your answers were very helpful for me, although my struggle with set theory is not over yet :). I do not choose accepted answer, as they all were helpful. | |
| Nov 3, 2010 at 13:06 | comment | added | David E Speyer | I voted to reopen. This strikes me as a reasonable question for a mathematician who has a good general background but has never actually learned rigorous set theory. In particular, the discussion about how Replacement fits in is not obvious. I don't know that this question needs any more answers -- Arturo's is very good -- but I don't agree that this sort of question is unwelcome. | |
| Nov 3, 2010 at 10:09 | history | closed |
Martin Brandenburg Andrés E. Caicedo Andreas Thom Pete L. Clark Andrew Stacey |
too localized | |
| Nov 3, 2010 at 4:07 | comment | added | Theo Johnson-Freyd | As @Stefan said, the trick is that by "family" we in particular mean set of sets. The idea is that "set"s are somehow guaranteed to be "small", as compared with "classes" or "universes" or whatever language you prefer. A good analogy is with numbers: there are infinite numbers, but any sum of finitely many finite numbers is guaranteed to be finite. Similarly, any union of "setly many" sets is a set. | |
| Nov 2, 2010 at 21:10 | comment | added | Stefan Geschke | The boundedness condition is just that the index class of the family is actually a set. | |
| Nov 2, 2010 at 20:49 | answer | added | arsmath | timeline score: 2 | |
| Nov 2, 2010 at 20:12 | answer | added | Arturo Magidin | timeline score: 8 | |
| Nov 2, 2010 at 20:11 | answer | added | Stefan Geschke | timeline score: 3 | |
| Nov 2, 2010 at 19:44 | comment | added | Kevin Buzzard | It's an axiom of ZF set theory that the union of a set of sets is a set. Does this help? | |
| Nov 2, 2010 at 19:37 | history | asked | Sasha | CC BY-SA 2.5 |