BZOJ2818: Gcd 莫比乌斯反演

#include<cstdio>
#include<cstring>
#include<queue>
#include<cstdlib>
#include<algorithm>
#include<vector>
#include<cmath>
using namespace std;
typedef long long LL;
const int N=1e7+5;
const int INF=0x3f3f3f3f;
bool vis[N];
int prime[N],mu[N],cnt;
void getmu()
{
    mu[1] = 1;
    for(int i=2; i<=N-5; i++)
    {
        if(!vis[i])
        {
            prime[++cnt] = i;
            mu[i] = -1;
        }
        for(int j=1; j<=cnt&&i*prime[j]<=N-5; j++)
        {
            vis[i*prime[j]] = 1;
            if(i%prime[j]) mu[i*prime[j]] = -mu[i];
            else
            {
                mu[i*prime[j]] = 0;
                break;
            }
        }
    }
}
int main(){
    getmu();
    for(int i=1;i<=N-5;++i)mu[i]+=mu[i-1];
    int n;
    scanf("%d",&n);
    LL ans=0;
    for(int i=1;i<=cnt&&prime[i]<=n;++i){
        int l=n/prime[i];
      for(int k=1,j;k<=l;k=j+1){
         j=l/(l/k);
        ans+=1ll*(mu[j]-mu[k-1])*(l/k)*(l/k);
      }
    }
    printf("%lld\n",ans);
    return 0;
}