题目链接：http://www.lydsy.com/JudgeOnline/problem.php?id=2818
Description
给定整数N，求1<=x,y<=N且Gcd(x,y)为素数的
数对(x,y)有多少对.
Input
一个整数N
Output
如题
懒得写题解就看黄学长的吧：http://hzwer.com/3466.html
这题不难
1 #include <iostream> 2 #include <cstdio> 3 #include <algorithm> 4 #include <cstring> 5 #include <cmath> 6 #define rep(i,l,r) for(int i=l; i<=r; i++) 7 #define clr(x,y) memset(x,y,sizeof(x)) 8 using namespace std; 9 typedef long long ll; 10 const int maxn = 10000010; 11 int n,cnt=0,pri[maxn],phi[maxn]; 12 ll sum[maxn]; 13 bool flag[maxn]; 14 void get_phi(){ 15 clr(flag,0); phi[1] = 1; 16 rep(i,2,n){ 17 if (!flag[i]) pri[++cnt] = i, phi[i] = i - 1; 18 for(int j = 1; j <= cnt && i*pri[j] <= n; j++){ 19 flag[i*pri[j]] = 1; 20 if (i % pri[j]) phi[i*pri[j]] = phi[i] * (pri[j] - 1); 21 else{ 22 phi[i*pri[j]] = phi[i] * pri[j]; 23 break; 24 } 25 } 26 } 27 sum[1] = phi[1]; rep(i,2,n) sum[i] = sum[i-1] + phi[i]; 28 } 29 int main(){ 30 scanf("%d",&n); 31 get_phi(); 32 ll ans = 0; 33 rep(i,1,cnt) ans += (sum[n/pri[i]] << 1) - 1; 34 printf("%lld ",ans); 35 return 0; 36 }