欧拉函数与欧拉定理

以下内容摘自acdreamer

 

求欧拉函数值

 

long long Euler(long long n)
{
    long long ans = n;
    for (long long i = 2; i * i <= n; i++)
    {
        if (n % i == 0)
        {
            ans = ans - ans / i;
            while (n % i == 0) n /= i;
        }
    }
    if (n > 1)
        ans = ans - ans / n;
    return ans;
}

void init()
{
    for (int i = 1; i < maxn; i++) Euler[i] = i;
    for (int i = 2; i < maxn; i += 2) Euler[i] >>= 1;
    for (int i = 3; i < maxn; i += 2)
    {
        if (Euler[i] == i)
        {
            for (int j = i; j < maxn; j += i)
                Euler[i] = Euler[i] - Euler[i] / i;
        }
    }
}