grad student wrote:
math dude wrote:Here is another question. Prove that there are more irrational than rational numbers.
I\\\\\\\'m sure I could have given you a more elegant proof during my course in real analysis, but here\\\\\\\'s a try:
(1) There are countably many rational numbers
(2) There are uncountably many real numbers in the interval (0,1)
(3) Therefore, since the union of two countable sets is countable (trivial proof), there are uncountably many irrational numbers in (0,1)
The proof for (1) isn\\\\\\\'t that hard, you just write the irrationals in matrix form, starting with 1/1 in the top left, and go like this:
1/1 1/2 1/3 1/4...
2/1 2/2 2/3 2/4...
3/1 3/2 3/3 3/4...
.
.
Then to count them you start at 1/1, go to 1/2, then 2/1, then 3/1, 2/2, 1/3, 1/4, 2/3, etc...snaking back and forth.
I\\\\\\\'ll leave (2) for someone else to prove.
Doesn\\\\\\\'t question 2 go something like:
Assume there are countably many real numbers in the interval (0,1).
So take any interval inside (0,1) such as (.259, .26), but we can find even more (uncountable) intervals inside (.259, .26).....
Yeah I know this is very crude but it has been a while.
I will throw this one out there as it was a favorite first day question of some of my professors.
Please prove that if a space is closed and bound that it is compact.