Euler totient function (phi) - YessineJallouli/Competitive-Programming GitHub Wiki
Generate Euler function :
#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int N = 1e5+7;
int phi[N];
int main() {
for (int i = 0; i < N; i++)
phi[i] = i;
for (int i = 1; i < N; i++) {
for (int j = 2 * i; j < N; j += i) {
phi[j] -= phi[i];
}
}
}