My teacher used some trick for proving there is no supremum in $\mathbb{Q}$ for $\{x|0 He defines a variable: $\xi=\frac{2c+2}{c+2}$. So, I noticed that $\xi=\sqrt{2}$ if $c=\sqrt{2}$. Where does this $\xi$ comes from?
Some Babylonian trick
3
$\begingroup$
real-analysis
-
0Possibe duplicate of http://math.stackexchange.com/questions/141774/choice-of-q-in-baby-rudins-example-1-1 – 2012-12-04
1 Answers
3
The expression for $\xi$ as $\frac{2c+2}{c+2}$ is just a convenient rational function of $c$ which is larger than $c$ when $0 This can then be used to get a strictly increasing sequence of values in the set $\{x \mid 0 There are many other possible expressions for $\xi$ as a function of $c$ which would work as well, so as to where this particular function comes from, I guess it might have been found by some Babylonian (although I didn't think they had algebra developed sufficiently, but I might be wrong!) Edit: Another example with similar properties that you might like to look at could be $\xi = \frac{3c+4}{2c+3}$.