jsfisher
ETcorngods survivor
- Joined
- Dec 23, 2005
- Messages
- 24,532
Jsfisher, set A is exactly only the particular numbers of the form 0.9,0.99,0.999, ... etc.
Yet this limited case can be used Without loss of generality in your proof by contradiction.
No, it cannot.
The thing to be proven is that the set {X : X<Y} has no largest value. However, since you keep mis-referring to it as a proof no real number has an immediate predecessor, and since that is substantially the same claim, I'll refer to it as such.
The proof introduces Y to represent an arbitrary real number.
By then fixing Y to be 1, as you did, you have now lost generality. If you continue the proof under this assumption, then all you end up proving is that 1 has no immediate predecessor.
Ok, so that's one fault. There is another.
The original proof uses the set {X : X<Y}, which is the set of all real numbers less then Y. More precisely, these are all the predecessors of Y. If there is an immediate predecessor, it must be in this set.
You, however, have restricted the set further, even though your notation is inadequate for the task. Instead of all predecessors, you've chosen just those that fit a tidy sequence (0.9, 0.99, 0.999, 0.9999, ... for the case at hand).
So, instead of all predecessors of Y, you've changed it to some predecessors of Y. Again, you have lost generality. All you will be able to prove with this restricted set is that the immediate predecessor of Y (if it exists) isn't in your restricted set.