数论：欧拉筛和莫比乌斯、欧拉函数

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int prim[Maxn],cnt,mu[Maxn],phi[Maxn];
bool vis[Maxn];
void Get(int n){
    mu[1] = phi[1] = 1;
    for(int i=2;i<=n;i++)
    {
        if( !vis[i] )  prim[++cnt] = i,mu[i] = -1,phi[i] = i-1;
        for(int j=1;j<=cnt&&i*prim[j]<=n;j++)
        {
            vis[i*prim[j]] = true;
            if( i%prim[j] == 0 )
            {
                phi[i*prim[j]] = phi[i]*prim[j];
                break;
            }
            mu[i*prim[j]] = -mu[i];
            phi[i*prim[j]] = phi[i]*(prim[j]-1);
        }
    }
}