GCD

Code:

#include<cstdio>              
using namespace std;
const int maxn=4000005;
const int R=4000002;
const int N=4000002;
long long sumv[maxn],f[maxn];
int phi[maxn],prime[maxn],vis[maxn];
void solve(){
	phi[1]=1;
	int cnt=0;
	for(int i=2;i<=R;++i){
		if(!vis[i])prime[++cnt]=i,phi[i]=i-1;
		for(int j=1;j<=cnt&&prime[j]*i<=R;++j){
			vis[prime[j]*i]=1;
			if(i%prime[j]!=0)phi[i*prime[j]]=phi[i]*(prime[j]-1);
			else{
				phi[i*prime[j]]=phi[i]*prime[j];
				break;
			}
		}
	}
	for(int i=1;i<N;++i)
		for(int j=i*2;j<N;j+=i)
			f[j]+=i*phi[j/i];
	for(int i=1;i<N;++i)
		sumv[i]=sumv[i-1]+f[i];
}
int main(){
	solve();
	while(1){
		int n;scanf("%d",&n);
		if(!n)break;
		printf("%lld
",sumv[n]);
	}
	return 0;
}