【BZOJ3518】点组计数 [欧拉函数]

 1 #include<iostream>  
 2 #include<string>  
 3 #include<algorithm>  
 4 #include<cstdio>  
 5 #include<cstring>  
 6 #include<cstdlib>  
 7 #include<cmath>
 8 using namespace std; 
 9 typedef long long s64;
10     
11 const int ONE = 50005;
12 const int MOD = 1000000007;
13 const int Ny6 = 166666668;
14 
15 int T;
16 int n,m;
17 bool isp[ONE];
18 int prime[ONE],p_num;
19 int phi[ONE];
20 s64 Ans;
21    
22 int get() 
23 {
24         int res=1,Q=1;  char c;
25         while( (c=getchar())<48 || c>57)
26         if(c==‘-‘)Q=-1;
27         if(Q) res=c-48; 
28         while((c=getchar())>=48 && c<=57) 
29         res=res*10+c-48; 
30         return res*Q; 
31 }
32 
33 void Getphi(int MaxN)
34 {
35         phi[1] = 1;
36         for(int i=2; i<=MaxN; i++)
37         {
38             if(!isp[i])
39                 prime[++p_num] = i, phi[i] = i - 1;
40             for(int j=1; j<=p_num, i*prime[j]<=MaxN; j++)
41             {
42                 isp[i * prime[j]] = 1;
43                 if(i % prime[j] == 0)
44                 {
45                     phi[i * prime[j]] = (s64)phi[i] * prime[j] % MOD;
46                     break;
47                 }
48                 phi[i * prime[j]] = (s64)phi[i] * phi[prime[j]] % MOD;
49             }
50         }
51 }
52 
53 s64 Get(int a,int b,int num) {return (s64)(a+b) * num / 2 %MOD; }
54 s64 Sum(int n,int m) {return ((s64)n*(n-1)/2%MOD) * ((s64)m*(m-1)/2%MOD) % MOD;}
55 
56 int C(int n)
57 {
58         int res = (s64)n * (n-1) % MOD * (n-2) % MOD;
59         return (s64) res * Ny6 % MOD;
60 }
61 
62 int main()
63 {
64         Getphi(ONE-1);
65         n=get();    m=get();
66         if(n > m) swap(n,m);
67         for(int d=1; d<=n; d++)
68         {
69             Ans += phi[d] * Get(n-d,n-(n/d)*d,n/d) % MOD *Get(m-d,m-(m/d)*d,m/d) % MOD;
70             Ans %= MOD;
71         }
72         
73         Ans = (Ans - Sum(n,m) + MOD) % MOD;
74         Ans = Ans*2%MOD + (s64)C(n)*m%MOD + (s64)C(m)*n%MOD;
75         Ans %= MOD;
76         
77         printf("%lld",Ans);
78 }