How can one show that Dedekind's cuts are equivalent to Cantor's definition of an irrational number based on Cauchy sequences? The problem hints that it might be useful to use the lemma that, for any positive integers (p,q), there exists a positive integer M, such that Mq > p.
In the way I tried to solve it, I started with the def. of Dedekind's cut: a < y < b, then said a = p/q, and b = M. Then I got y - p/q > 0, and y - M < 0. Then, one can choose p/q and y to be such that the module of the inequalities above will be < epsilon, and I guess this would satisfy a Cauchy's sequence requirement. However, I am definitely not very happy with this solution, especially with the last steps.
Any hints would be useful.