Does there exist a sequence of pairwise distinct integers that satisfies both of the following conditions?
a. For all positive integers , we have and .
b. For all positive integers , we have .
Does there exist a sequence of pairwise distinct integers that satisfies both of the following conditions?
a. For all positive integers , we have and .
b. For all positive integers , we have .
Proof. Such a sequence does not exist. We prove this by contradiction. Suppose there exists such a sequence. Take a positive integer satisfying
Such an exists because
which can be arbitrarily large.
We prove that at least elements of fall into the interval , which contradicts the assumption that 's are all distinct. To show this, it suffices to prove that for ,
(i) At least elements of fall into .
(ii) At least elements of fall into .
In this way, the total number of elements in is greater than or equal to
We will only prove (i), and the proof of (ii) is similar. Note that for , we have
We consider the following three cases.
(a) If there exists an in the sequence , then (*) implies that there are at least elements in among .
(b) If there exists an in the sequence , then (*) implies that there are at least elements in among .
(c) If neither (a) nor (b) holds, then all the elements in the sequence are within , and there are a total of elements.
This completes the proof of (i), and the proof of (ii) can be similarly established. Therefore, a contradiction is reached, and thus such a sequence does not exist.