COIN（欧拉函数）

Posted 2020-12-27 aze-qwq

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cmath>
 4 using namespace std;
 5 typedef long long ll;
 6 ll x, ans=0;
 7 int euler[10000005];
 8 void e(int n){
 9     euler[1]=1;
10     for(int i=2; i<=n; i++) euler[i]=i;
11     for(int i=2; i<=n; i++)
12         if(euler[i]==i)
13                for(int j=i; j<n; j+=i)
14                   euler[j]=euler[j]/i*(i-1);
15 }
16 int main(){
17     //freopen("coin.in","r",stdin);
18     //freopen("coin.out","w",stdout);
19     cin>>x;
20     ll up=sqrt(x);
21     e(up+1);
22     for(ll i=1; i<=up; i++){
23         ans+=euler[i]*floor(x/(i*i));
24     }
25     cout<<ans; 
26     //fclose(stdin);
27     //fclose(stdout);
28     return 0;
29 }