- #1

Bashyboy

- 1,421

- 5

(a) If ##x \in R##, ##y \in R##, and ##x > 0##, then there exists a positive integer ##n## such that ##nx > y##.

(a) Let A be the set of all nx. If (a) were false, then y would be an upper bound of A.

**But then A has a least upper bound in R**...

I understand the method of proof he is employing, a proof by contradiction, which leads to y being larger than every element A (hence, it is an upper bound). However, I do not understand how this immediately implies that A

*must*have a least upper bound. Which theorem is he invoking to come to such a conclusion?

EDIT: Also, what is the quantification on x and y?