[素数]线性筛-欧拉函数

int tot = 0;
memset(check, 0, sizeof(check));
for (int i = 2; i < MAXL; ++i)
{
  if (!check[i])
  {
    prime[tot++] = i;
  }
  for (int j = 0; j < tot; ++j)
  {
    if (i * prime[j] > MAXL)
    {
      break;
    }
    check[i*prime[j]] = 1;
    if (i % prime[j] == 0)
    {
      break;
    }
  }
}

欧拉函数：在数论，对正整数n，欧拉函数是小于或等于n的数中与n互质的数的数目。

#include<cstdio>
#include<cstring>
#define MAXN 100005
#define MAXL 1299710
int prime[MAXN];
int check[MAXL];
int phi[MAXL];
int tot = 0;
phi[1] = 1;
memset(check, 0, sizeof(check));
for (int i = 2; i < MAXL; ++i)
{
  if (!check[i])
  {
    prime[tot++] = i;
    phi[i] = i - 1;
  }
  for (int j = 0; j < tot; ++j)
  {
    if (i * prime[j] > MAXL)
    {
      break;
    }
    check[i*prime[j]] = 1;
    if (i % prime[j] == 0)
    {
      phi[i*prime[j]] = phi[i] * prime[j];
      break;
    }else
    {
      phi[i*prime[j]] = phi[i] * (prime[j]-1);
    }
  }
}

int f[N],prime[N],pre[M*10];
int cnt;
void init(){
    f[1]=1;
    for(int i=2;i<=1e7;i++){
        if(prime[i]==0){
            pre[++cnt]=i;
            f[i]=i-1;
        }
        for(int j=1;j<=cnt&&pre[j]*i<=1e7;j++){
            prime[i*pre[j]]=1;
            f[i*pre[j]]=f[i]*f[pre[j]];
            if(i%pre[j]==0){
                f[i*pre[j]]=f[i]*(f[pre[j]]+1);
                break;
            }
        }
    }
}