昨天看了iwtwiioi的推导感觉很神！
今天想写结果又忘了怎么推的啦。。。233
1 /************************************************************** 2 Problem: 2154 3 User: rausen 4 Language: C++ 5 Result: Accepted 6 Time:5124 ms 7 Memory:127760 kb 8 ****************************************************************/ 9 10 #include <cstdio> 11 #include <algorithm> 12 13 using namespace std; 14 typedef long long ll; 15 16 const int N = 1e7 + 5; 17 const ll mod = 20101009; 18 19 int n, m; 20 int p[N], cnt; 21 bool vis[N]; 22 ll g[N]; 23 24 void get_g(int N) { 25 int i, j, k; 26 g[1] = 1; 27 for (i = 2; i <= N; ++i) { 28 if (!vis[i]) p[++cnt] = i, g[i] = 1 - i; 29 for (j = 1; j <= cnt; ++j) { 30 if ((k = i * p[j]) > N) break; 31 vis[k] = 1; 32 if (i % p[j] == 0) { 33 g[k] = g[i]; 34 break; 35 } 36 g[k] = g[i] * (1 - p[j]); 37 } 38 } 39 for (i = 2; i <= N; ++i) g[i] *= i; 40 for (i = 1; i <= N; ++i) 41 (g[i] += g[i - 1]) %= mod; 42 } 43 44 ll calc(int n, int m) { 45 int i, j; 46 ll t1, t2, res = 0; 47 for (i = 1, j = 0; i <= n; i = j + 1) { 48 j = min(n / (n / i), m / (m / i)); 49 t1 = (1ll * (n / i) * (n / i + 1) / 2) % mod; 50 t2 = (1ll * (m / i) * (m / i + 1) / 2) % mod; 51 (res += ((g[j] - g[i - 1]) * ((t1 * t2) % mod)) % mod) %= mod; 52 } 53 return (res % mod + mod) % mod; 54 } 55 56 int main() { 57 int i, T; 58 scanf("%d%d", &n, &m); 59 if (n > m) swap(n, m); 60 get_g(m); 61 printf("%lld ", calc(n, m)); 62 return 0; 63 }