wenbao与数论（大白书）

最大公约数之和

https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&category=26&problem=2421&mosmsg=Submission+received+with+ID+18788108

输入n 输出gcd（i，j） 1<=i<j<=n总和

#include <iostream>
using namespace std;
#define REP(i, x, n) for(int i = x; i < n; i++)
#define ll long long
const int maxn = 4000005;
int phi[maxn], p[maxn];
ll f[maxn], s[maxn];
bool vis[maxn];
void Euler(int n){
    int i, j, k, cnt = 0;
    phi[1] = 1;
    for (i = 2; i < n; ++i){
        if (!vis[i]){
            p[cnt++] = i;
            phi[i] = i - 1;
        }
        for (j = 0; j < cnt && i * p[j] < n; ++j){
            vis[i * p[j]] = true;
            if (i % p[j]) phi[i * p[j]] = phi[i] * phi[p[j]];
            else {
                phi[i * p[j]] = phi[i] * p[j];
                break;
            }
        }
    }
}
int main(){
    Euler(maxn);
    REP(i, 1, maxn){
        for(int j = i*2; j < maxn; j += i){
            f[j] += i * phi[j/i]; 
        }
    }
    s[2] = f[2];
    REP(i, 3, maxn) s[i] = s[i-1] + f[i];
    int n;
    while(scanf("%d", &n) && n){
        printf("%lld
", s[n]);
    }
    return 0;
}

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