1968: [Ahoi2005]COMMON 约数研究

Posted 2020-06-25 xiyuedong

 1 #include<cstdio>
 2 #include<iostream>
 3 #define M 1000008
 4 using namespace std;
 5 long long tot,f[M],sum[M],ans,n,b[M],p[M];
 6 int main()
 7 {
 8     scanf("%d",&n);
 9     ans=1;
10     for(int i=2;i<=n;i++)
11       {
12         if(!b[i])
13           {
14             tot++;
15             p[tot]=i;
16             sum[i]=1;
17             f[i]=2;
18           }
19         for(int j=1;j<=tot;j++)
20           {
21             if(i*p[j]>n)
22               break;
23             b[i*p[j]]=1;
24             if(i%p[j])
25               {
26                 f[i*p[j]]=f[i]*2;
27                 sum[i*p[j]]=1;
28               }
29             else
30               {
31                 f[i*p[j]]=f[i]/(sum[i]+1)*(sum[i]+2);
32                 sum[i*p[j]]=sum[i]+1;
33               }
34           }
35         ans+=f[i];
36       }
37     printf("%lld\n",ans);
38     return 0;
39 }

权当练习下线性筛求约数和。