Thursday, 24 September 2009

set theory - Why is it important to have disjoint sets in a union for the union to make sense w.r.t the order types?

This question has been bugging me for quite some time now.



Say we have some beta smaller than some gamma and a sequence



betaepsilon : epsilon smaller than cf(beta) cofinal in beta and say



we have some sets Anepsilon and each of these Anepsilon has order type less than gamman.



Now foralln in omega let Bn= cup Anepsilon for all epsilon < gamma and suppose in the end I can write beta as the union of all the Bn (but that is not really my problem here)



Why can I deduce that Bn has order type less than gamman+1only if all my sets Anepsilon are disjoint and do not overlap?



(since we have a union of less then gamma sets each of which is of order type less than gamman)



Why can't I still guarantee that the Bn will still have order type gamman+1 if all the Anepsilon are not disjoint?



I know that I need to take the Anepsilon to be [epsilon,epsilon+1) so that they are disjoint.



But why does everything in the union have to be in order?



I hope I conveyed my question clearly. Thanks in advance for any help.

No comments:

Post a Comment