Tuesday, 10 September 2013

analytic number theory - Asymptotic behaviour of omega(n)



I've been working on this homework exercise for quite some time now and I just won't succeed. Any help would be appreciated.



Let ω(n) be the number of distinct prime divisors of n. I need to prove that ω(n)=O(log(n)loglog(n)) as x. There is a hint which says to first prove ω(n)!n (which was easy) . Furthermore, I may use ω(n)!exp(ω(n)log(ω(n))ω(n)).



From these hints it follows that ω(n)log(ω(n))ω(n)log(n) and thus ω(n)log(n)/(log(ω(n))1), but then I get stuck.



Alternatively, ω(n)log(ω(n))log(n)+ω(n)2log(n) and I get stuck again.




I've found a proof which I understand but it does not use any of the hints. Since I have been giving this some time now, I would really like to know how to do it in the way suggested by the exercise.



Thanks in advance!


Answer



Once you have the inequality ω(n)logω(n)2logn you're basically there. Now assume for contradiction that for all large enough n we have ω(n)>Clognloglogn. Here C is some large constant, though actually C>2 suffices.


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