题目链接

https://www.luogu.org/problemnew/show/P4450

题解

直接反演一下就好了,甚至都不用整除分块……

代码

#include <cstdio>
#include <algorithm>

int read()
{
  int x=0,f=1;
  char ch=getchar();
  while((ch<'0')||(ch>'9'))
    {
      if(ch=='-')
        {
          f=-f;
        }
      ch=getchar();
    }
  while((ch>='0')&&(ch<='9'))
    {
      x=x*10+ch-'0';
      ch=getchar();
    }
  return x*f;
}

const int maxn=1000000;

int p[maxn+10],prime[maxn+10],cnt,mu[maxn+10];

int getprime()
{
  p[1]=mu[1]=1;
  for(int i=2; i<=maxn; ++i)
    {
      if(!p[i])
        {
          prime[++cnt]=i;
          mu[i]=-1;
        }
      for(int j=1; (j<=cnt)&&(i*prime[j]<=maxn); ++j)
        {
          int x=i*prime[j];
          p[x]=1;
          if(i%prime[j]==0)
            {
              mu[x]=0;
              break;
            }
          mu[x]=-mu[i];
        }
    }
  return 0;
}

int n,m,k;
long long ans;

int main()
{
  getprime();
  n=read();
  m=read();
  k=read();
  for(int i=1; i<=std::min(n,m)/k; ++i)
    {
      ans+=1ll*mu[i]*(n/(k*i))*(m/(k*i));
    }
  printf("%lld\n",ans);
  return 0;
}