notebook/code/Euler.cpp

const int maxn = 100000;
// Compute phi(n);
int euler_phi(int n) {
	int m = (int) sqrt(n + 0.5);
	int ans = n;
	for(int i = 2; i <= m; i++)
    if(n % i ==0) {
	    ans = ans / i * (i-1);
      while(n % i ==0)
        n /= i;
     }
	if(n > 1)
    ans = ans / n * (n-1);
	return ans;
}
// Compute all phi(1), phi(2), ... , phi(n)
int phi[maxn];
void phi_table(int n) {
	for(int i = 2; i <= n; i++)
    phi[i] = 0;
	phi[1] = 1;
	for(int i = 2; i <= n; i++)
    if(!phi[i])
  		for(int j = i; j <=n; j += i) {
  			if(!phi[j]) phi[j] = j;
  			phi[j] = phi[j] / i *(i-1);
  		}
}