bzoj 2190

开始补我不敢碰的数论QAQ

看了资料感觉还是很晕，先把结论记下吧。。

若(x,y)符合提议gcd(x-1,y-1)=1，那么先欧拉筛法求出s=sigma(phi[i])，然后注意(1,2)(2,1)(2,2)3个点，答案就是2*s+1了

#include<bits/stdc++.h>
#define inc(i,l,r) for(int i=l;i<=r;i++)
#define dec(i,l,r) for(int i=l;i>=r;i--)
#define link(x) for(edge *j=h[x];j;j=j->next)
#define mem(a) memset(a,0,sizeof(a))
#define inf 1e9
#define ll long long
#define succ(x) (1<<x)
#define NM 40000+5
using namespace std;
int read(){
    int x=0,f=1;char ch=getchar();
    while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
    while(isdigit(ch))x=x*10+ch-'0',ch=getchar();
    return x*f;
}
int phi[NM],p[NM],n,tot,check[NM];
ll s;
int main(){
    freopen("data.in","r",stdin);
    n=read();
    s=phi[1]=1;
    inc(i,2,n-1){
        if(!check[i]){
            p[++tot]=i;
            phi[i]=i-1;
        }
        s+=phi[i];
        inc(j,1,tot){
            if(i*p[j]>n)break;
            check[i*p[j]]++;
            if(i%p[j])phi[i*p[j]]=phi[i]*phi[p[j]];
            else{
                phi[i*p[j]]=phi[i]*p[j];
                break;
            }
        }
    }
//    inc(i,1,n)printf("%d ",phi[i]);printf("\n");
    printf("%lld\n",s*2+1);
    return 0;
}