題解：歐拉函數

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;

int n;
int ans=0;

int phi[100000];
int vis[100000]={0};
int prime[100000],cntprime=0;
int Lineshake(){
    vis[1]=1;phi[1]=1;
    for(int i=2;i<=n;++i){
        if(!vis[i]){
            prime[++cntprime]=i;
            phi[i]
 
=i-1;
        }
        for(int j=1;(j<=cntprime)&&(i*prime[j]<=n);++j){
            vis[i*prime[j]]=1;
            if(i%prime[j]==0){
                phi[i*prime[j]]=phi[i]*prime[j];
                break;
            }
            phi[i*prime[j]]=phi[i]*(prime[j]-1);
        }
    }
}

 
int main(){
    scanf("%d",&n);
    Lineshake();
    for(int i=2;i<=n-1;++i)ans=ans+phi[i]*2;
    printf("%d\n",ans+3);
    return 0;
}

BZOJ：2190: [SDOI2008]儀仗隊