- #1
madilyn
- 13
- 0
Homework Statement
Let [itex]f(n)[/itex] denote the number of unique prime factors of some positive integer [itex]n > 1[/itex]. Prove that [itex]f(n) \in \mathcal{O}\left(\dfrac{\log n}{\log \log n}\right)[/itex]
Homework Equations
The Attempt at a Solution
Since every prime number except 2 is prime, an upper bound on the number of prime factors of any [itex]n[/itex] would be [itex]k[/itex] such that
[tex]\displaystyle{\dfrac{n}{\prod_{i=1}^{k}\left(2i-1\right)}=1}[/tex]
[tex]\ln n=\sum_{i=1}^{k}\ln\left(2i-1\right)[/tex]
Using AM-GM inequality,
[tex]\sum_{i=1}^{k}\ln\left(2i-1\right)\geq k\sqrt[k]{\prod_{i=1}^{k}\left(2i-1\right)}=k\sqrt[k]{n}[/tex]
The best I can arrive at is:
[tex]\ln\ln n\geq\ln k+\dfrac{1}{k}\ln n[/tex]
which is ugly. I've also tried attacking this with
[tex]\dfrac{\sqrt{n}}{\prod_{i=1}^{k}\left(2i-1\right)}=1[/tex]
instead, but this makes my inequality uglier.
What should I do? Thanks!