CF284A Cows and Primitive Roots

嘟嘟嘟

这道题就是求一个奇素数\(p\)的原根数量。 　　

公式是\(\varphi(\varphi(p))\)。又因为\(p\)是质数，所以就是\(\varphi(p - 1)\)。

（证明啥的我不会……）

#include<cstdio>
#include<iostream>
#include<cmath>
#include<algorithm>
#include<cstring>
#include<cstdlib>
#include<cctype>
#include<vector>
#include<stack>
#include<queue>
using namespace std;
#define enter puts("")
#define space putchar(' ')
#define Mem(a, x) memset(a, x, sizeof(a))
#define rg register
typedef long long ll;
typedef double db;
const int INF = 0x3f3f3f3f;
const db eps = 1e-8;
//const int maxn = ;
inline ll read()
{
  ll ans = 0;
  char ch = getchar(), last = ' ';
  while(!isdigit(ch)) last = ch, ch = getchar();
  while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();
  if(last == '-') ans = -ans;
  return ans;
}
inline void write(ll x)
{
  if(x < 0) x = -x, putchar('-');
  if(x >= 10) write(x / 10);
  putchar(x % 10 + '0');
}

int n;
int phi(int n)
{
  int ans = n;
  for(int i = 2; i * i <= n; ++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;
}

int main()
{
  while(scanf("%d", &n) != EOF) write(phi(n - 1)), enter;
  return 0;
}