poj2478

欧拉函数，n每扩大一个，就会有euler(n)个数字加入到集合中，所以欧拉函数的前n项和即为所求。

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

#define maxn 1000006

long long phi[maxn], num[maxn];
int n;

int main()
{
    //freopen("t.txt", "r", stdin);
    int i, j;
    for (i = 1; i <= maxn; i++)
        phi[i] = i;
    for (i = 2; i <= maxn; i += 2)
        phi[i] /= 2;
    for (i = 3; i <= maxn; i += 2)
        if (phi[i] == i)
        {
            for (j = i; j <= maxn; j += i)
                phi[j] = phi[j] / i * (i - 1);
        }
    num[0] = 0;
    for (int i = 1; i < maxn; i++)
        num[i] = num[i - 1] + phi[i];
    while (scanf("%d", &n), n != 0)
    {
        printf("%lld\n", num[n] - 1);
    }
    return 0;
}