Use of Euler's function

Reprinted from https://www.cnblogs.com/handsomecui/p/4755455.html
Euler function template
Euler function must be an even number
(1) Directly find the number of less than or equal to n and relatively prime to n:

int Euler(int n)

{

int ret=n;

for(int i=2;i<=sqrt(n);i++)

 if(n%i==0)

  {

    ret=ret/i*(i-1);//先进行除法防止溢出(ret=ret*(1-1/p(i)))

    while(n%i==0)

      n/=i;

 }

if(n>1)

      ret=ret/n*(n-1);

    return ret;

}
//
ll phi(ll n) // find Euler function value
{
int ans=n,temp=n;
for(int i=2;i*i<=temp;i++)
{
if(temp%i0)
{
ans-=ans/i;
while(temp%i 0) temp/=i;
}
}
if(temp>1) ans-=ans/temp;
return ans;
}

Screening template: find the number of prime factors of each number between [1,n]

#define size 1000001

int euler[size];

void Init()

{

 memset(euler,0,sizeof(euler));

      euler[1]=1;

 for(int i=2;i<size;i++)

   if(!euler[i])

   for(int j=i;j<size;j+=i)

   {

          if(!euler[j])

           euler[j]=j;

           euler[j]=euler[j]/i*(i-1);//先进行除法是为了防止中间数据的溢出

     }

}