UVa 11426 (欧拉函数 GCD之和) GCD - Extreme (II)

 1 #include <cstdio>
 2 typedef long long LL;
 3 
 4 const int maxn = 4000000;
 5 
 6 int phi[maxn + 10];
 7 LL f[maxn + 10];
 8 
 9 void phi_table()
10 {
11     phi[1] = 1;
12     for(int i = 2; i <= maxn; i++) if(!phi[i])
13         for(int j = i; j <= maxn; j += i)
14         {
15             if(!phi[j]) phi[j] = j;
16             phi[j] = phi[j] / i * (i-1);
17         }
18 }
19 
20 int main()
21 {
22     phi_table();
23 
24     for(int i = 1; i <= maxn; i++)
25         for(int j = i*2; j <= maxn; j += i)
26             f[j] += i * phi[j / i];
27     for(int i = 3; i <= maxn; i++) f[i] += f[i - 1];
28 
29     int n;
30     while(scanf("%d", &n) == 1 && n) printf("%lld\n", f[n]);
31 
32     return 0;
33 }