bzoj2005: [Noi2010]能量采集

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cmath>
using namespace std;
typedef long long LL;

LL pr,prime[110000],u[110000];
bool v[110000];
void mobius_inversion()
{
    pr=0;u[1]=1;
    memset(v,true,sizeof(v));
    for(int i=2;i<=100000;i++)
    {
        if(v[i]==true)
        {
            prime[++pr]=i;
            u[i]=-1;
        }
        for(int j=1;j<=pr&&i*prime[j]<=100000;j++)
        {
            v[i*prime[j]]=false;
            if(i%prime[j]==0){u[i*prime[j]]=0;break;}
            u[i*prime[j]]=-u[i];
        }
        u[i]+=u[i-1];
    }
}

LL getFi(LL n,LL m)
{
    LL t=min(n,m),last;LL Fi=0;
    for(LL d=1;d<=t;d=last+1)
    {
        last=min(n/(n/d),m/(m/d));
        LL Gd=(n/d)*(m/d);
        Fi+=(u[last]-u[d-1])*Gd;
    }
    return Fi;
}
int main()
{
    mobius_inversion();
    
    LL n,m;
    scanf("%lld%lld",&n,&m);
    if(n>m)swap(n,m);
    
    LL ans=n*m;
    for(int i=1;i<=n;i++)
    {
        ans+=(i-1)*2*getFi(n/i,m/i);
    }
    printf("%lld\n",ans);
    return 0;
}