【SPOJ】【P5971】【LCM Sum】【题解】【数论】

#include<cstdio>
#include<iostream>
#include<algorithm>
using namespace std;
int getint(){
int res=0;char c=getchar();
while(!isdigit(c))c=getchar();
while(isdigit(c))res=res*10+c-'0',c=getchar();
return res;
}
typedef long long LL;
const int maxn=1e6+5;
bool p[maxn];
int prime[maxn];
int phi[maxn];
LL sum[maxn];
LL anss[maxn];
void getphi(){
phi[1]=1;
for(int i=2;i<maxn;i++){
if(!p[i]){
prime[++prime[0]]=i;
phi[i]=i-1;
}for(int j=1;i*prime[j]<maxn&&j<=prime[0];j++){
p[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);
}
}for(int i=1;i<maxn;i++)sum[i]=(LL)i*phi[i];
for(int i=1;i<maxn;i++)
for(int j=i;j<maxn;j+=i)
anss[j]+=sum[i];
}
int main(){
int T=getint();
getphi();
while(T--){
int n=getint();
LL ans=(anss
+1)*n/2;
printf("%lld\n",ans);
}
return 0;
}