1 #include<iostream> 2 #include<stdio.h> 3 #include<cstring> 4 #include<cstdlib> 5 using namespace std; 6 typedef __int64 LL; 7 const LL mod = 1000000007; 8 LL cnm[5002][5002]; 9 LL dp[5002][5002]; 10 11 void prepare() 12 { 13 LL i,j; 14 for(i=0;i<=5000;i++) 15 { 16 cnm[0][i]=1; 17 cnm[i][0]=1; 18 } 19 for(i=1;i<=5000;i++) 20 { 21 for(j=1;j<=i&&j<=5000;j++) 22 { 23 if(i==j) { cnm[i][j]=1;continue;} 24 if(j==1) { cnm[i][j]=i;continue;} 25 cnm[i][j]=(cnm[i-1][j]+cnm[i-1][j-1])%mod; 26 } 27 } 28 dp[0][0]=1; 29 for(i=1;i<=5000;i++) 30 for(j=1;j<=5000;j++) 31 dp[i][j]=(dp[i-1][j-1]+((i-1)*dp[i-1][j])%mod)%mod; 32 } 33 int main() 34 { 35 LL n,a,b,cur; 36 LL l,r; 37 prepare(); 38 while(scanf("%I64d%I64d%I64d",&n,&a,&b)>0) 39 { 40 if(n==0&&a==0&&b==0)break; 41 l=a;r=n-b+1; 42 for(cur=0;l<=r;l++) 43 { 44 cur=( cur+((cnm[n-1][l-1]*dp[l-1][a-1])%mod)*dp[n-l][b-1])%mod; 45 } 46 printf("%I64d ",cur); 47 } 48 return 0; 49 }