题目描述

Eddy has solved lots of problem involving calculating the number of coprime pairs within some range. This problem can be solved with inclusion-exclusion method. Eddy has implemented it lots of times. Someday, when he encounters another coprime pairs problem, he comes up with diff-prime pairs problem. diff-prime pairs problem is that given N, you need to find the number of pairs (i, j), where $i \over gcd(i,j)$ and $j \over gcd(i, j)$ are both prime and i ,j ≤ N. gcd(i, j) is the greatest common divisor of i and j. Prime is an integer greater than 1 and has only 2 positive divisors.
Eddy tried to solve it with inclusion-exclusion method but failed. Please help Eddy to solve this problem.
Note that pair (i1, j1) and pair (i2, j2) are considered different if i1 ≠ i2 or j1 ≠ j2.

思路

$\sum_{i=1}^nprimecnt[\lfloor {n \over i}\rfloor]*(primecnt[\lfloor {n \over i}\rfloor]-1)$ 即为答案

❤采之欲遗谁，所思在远道。❤
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