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  • 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;
    }
    
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  • 原文地址:https://www.cnblogs.com/guangheli/p/9845074.html
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