HDU3501 Calculation 2（欧拉函数）

 1 #include<cstdio>
 2 #include<cstring>
 3 using namespace std;
 4 int phi(int n){
 5     int res=n;
 6     for(int i=2; i*i<=n; ++i){
 7         if(n%i) continue;
 8         while(n%i==0) n/=i;
 9         res-=res/i;
10     }
11     if(n!=1) res-=res/n;
12     return res;
13 }
14 int main(){
15     int n;
16     while(~scanf("%d",&n) && n){
17         printf("%lld\n",((long long)n*(n-1)/2-(long long)n*phi(n)/2)%1000000007);
18     }
19     return 0;
20 }