the question is very simple we just need to calculate total number of numbers which have exactly 4 divisors for ex 6, 8, 10 these are all of the forms p^3 or p*q but here n<=10^11
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the question is very simple we just need to calculate total number of numbers which have exactly 4 divisors for ex 6, 8, 10 these are all of the forms p^3 or p*q but here n<=10^11
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Auto comment: topic has been updated by shrohit_007 (previous revision, new revision, compare).
Here $$$\pi(n)$$$ denotes prime counting function, i.e number of primes not greater than $$$n$$$.
Numbers form $$$p^3$$$ can be easily counted, it's just $$$\pi \left(\lfloor \sqrt[3]{n} \rfloor\right)$$$.
To calculate numbers form $$$pq$$$ recall, that
.
Primes and $$$\pi$$$ up to $$$\sqrt{n}$$$ can be calculated straightforward using Eratosphenes sieve.
Also you can calculate values of $$$\pi\left( \lfloor \frac{n}{k} \rfloor \right)$$$ for all $$$k \geqslant 1$$$ using $$$O(n^{2/3})$$$ time as described here https://codeforces.net/blog/entry/91632
IMHO this question is not very simple as it requires some knowledge.