题目：https://www.luogu.com.cn/problem/P9238

思路：

代码：

#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<string>
#include<cstring>
#include<cmath>
#include<ctime>
#include<algorithm>
#include<utility>
#include<stack>
#include<queue>
#include<vector>
#include<set>
#include<map>
#include<unordered_map>
using namespace std;
#define  LL long long 
const int N = 1e7+ 10;
const double PI = acos(-1);
LL mod = 998244353;
LL n, mu[N], su[N], a[N], cn;
unordered_map<LL, LL> mpu;
void init()
{
    mu[1] = a[1] = 1;
    for (int i = 2; i < N; i++)
    {
        if (!a[i]) su[++cn] = i, mu[i] = -1;
        for (int j = 1; (LL)su[j] * i < N; j++)
        {
            LL mp = su[j];
            a[i * mp] = 1;
            if (i % mp == 0)
            {
                mu[mp * i] = 0;
                break;
            }
            mu[i * mp] = mu[i] * -1;
        }
    }
    for (int i = 1; i < N; i++) mu[i] += mu[i - 1];
}
LL seekmu(LL n)
{
    if (n < N) return mu[n];
    if (mpu[n]) return mpu[n];
    LL ans = 1;
    for (LL l = 2, r; l <= n; l = r + 1)
    {
        r = n / (n / l);
        ans -=(r-l+1)*seekmu(n/l);
    }
    mpu[n] = ans;
    return ans;
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    init();
    LL ans = 0;
    cin >> n;
    LL m = sqrtl(n);
    for (LL l = 1, r; l <= m; l = r + 1)
    {
        r = sqrtl(n / (n / (l*l)));
        ans += (n / (l * l))*(seekmu(r) - seekmu(l-1));
    }
    cout << ans << endl;
    return 0;
}