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题目：给你一个N，求sigma(gcd(i,j))，1<=i<j<=N

思路：枚举gcd，利用欧拉函数计算

代码：

#pragma comment(linker, "/STACK:1024000000,1024000000")#include<iostream>#include<algorithm>#include<ctime>#include<cstdio>#include<cmath>#include<cstring>#include<string>#include<vector>#include<map>#include<set>#include<queue>#include<stack>#include<list>#include<numeric>using namespace std;#define LL long long#define ULL unsigned long long#define INF 0x3f3f3f3f#define mm(a,b) memset(a,b,sizeof(a))#define PP puts("*********************");template<class T> T f_abs(T a){ return a > 0 ? a : -a; }template<class T> T gcd(T a, T b){ return b ? gcd(b, a%b) : a; }template<class T> T lcm(T a,T b){return a/gcd(a,b)*b;}// 0x3f3f3f3f3f3f3f3f// 0x3f3f3f3fconst int maxn=1e6+50;int phi[maxn];LL ans[maxn];void phi_table(int n){    for(int i=1;i<=n;i++)        phi[i]=i;    for(int i=2;i<=n;i+=2)        phi[i]/=2;    for(int i=3;i<=n;i+=2)        if(phi[i]==i){            for(LL j=i;j<=n;j+=i)                phi[j]=phi[j]/i*(i-1);        }    for(int i=1;i<=n;i++)        ans[i]=phi[i];    for(int i=2;i*i<=n;i++){        ans[i*i]+=(LL)phi[i]*i;        for(int j=i+1;j*i<=n;j++)            ans[j*i]+=(LL)phi[i]*j+(LL)phi[j]*i;    }    ans[1]=0;    for(int i=2;i<=n;i++)        ans[i]+=ans[i-1];}int main(){    int n;    phi_table(maxn-50);    while(~scanf("%d",&n)){        if(!n) break;        printf("%lld\n",ans[n]);    }    return 0;}

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