gcd(1,n)+gcd(2,n)....gcd(n-1,n); Uva11426

Posted 2021-01-14 andromeda-galaxy

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int maxn=4e6+100;
int phi[maxn];
int prime[maxn];
int visit[maxn];
int tot=0;
int num[maxn];
int ans[maxn];
void build_phi(int n)
{
    phi[1]=1;
    for(int i=2;i<=n;i++)
    {
        if(!visit[i])
        {
          prime[++tot]=i;
           phi[i]=i-1;
        }
        for(int j=1;j<=tot && i*prime[j]<=n;j++)
        {
            visit[i*prime[j]]=1;
            if(i%prime[j]==0)
            {
                phi[i*prime[j]]=phi[i]*prime[j]; break;
            }
            else
            {
                phi[i*prime[j]]=phi[i]*(prime[j]-1);
            }
        }
    }
}
void solve(int n)
{
        for(int i=1;i<=n;i++)
          for(int j=i*2;j<=n;j+=i)
            num[j] += phi[j/i]*i;
        for(int i=2;i<=n;i++)
        {
            ans[i]=ans[i-1]+num[i];
        }

}
int32_t main()
{
    build_phi(maxn);
    solve(maxn);
    //cout<<phi[1000000]<<endl;
    int n;
    while(cin>>n&&n)
    {
        cout<<ans[n]<<endl;
    }
    return 0;
}