Description

Calculate:

$$\sum _{x=0}^N{C}_N^x\cdot x^k$$

• $1⩽N⩽{10}^9,1⩽k⩽5000$

Prefix Knowledge

• Sterling numbers the second kind

  Sterling numbers the second kind, which is written in $S(n,k)$, is defined as the number of ways to put $n$ different balls into $k$ same boxes.

  It can be calculated as below:

  $$\begin{array}{rl}S(1,1)& =1\\ S(n,k)& =S(n-1,k-1)+n\cdot S(n-1,k)\end{array}$$

Solution

Use Stirling numbers the second kind to change the form of $x^k$

We have:

Proof:

Let's consider a problem: What's the number of ways to put $k$ different balls into $x$ different boxes.

Obviously the answer is $x^k$.

In another way, we may index $i$ that represents the number of boxes that have balls in, then the ways in this case is $S(k,i)$. And because the boxes are different , we should multiply $i!$

Thus, the answer is also $\sum _{i=0}^x{C}_x^i\cdot i!\cdot S(k,i)$

So now we have $x^k=\sum _{i=0}^x{C}_x^i\cdot i!\cdot S(k,i)$

Then we have:

Then change the form of $\sum _{x\in [i,n]}{C}_N^x\cdot C_x^i\cdot i!$

Notice that this is the answer to the problem:

What's the number of ways to choose $x$ objects(same) from $N$, then $i$ (different) from $k$ ?

We may first index $i$, then consist the other $N-i$. They can be either chosen or not during the first round.

Thus we have $\sum {x\in[i,N]}C {N}^x\cdot C x^i\cdot i!=A {N}^i\cdot 2^{N-i}$

After,

We may notice that when $i⩾k,S(k,i)=0$, so we just need to index $i$ from $0$ to $k$, not to $N$

Thus, the problem can be solved in $O(k^2)$

Code

const int N = 5005;
const int MOD = 1e9+7;
int s[N][N];
int n, k;
int power(int base, int exp) {
   if (exp < 0) return 0;
   int res=1;
   while (exp) {
       if (exp&1) (res*=base)%=MOD;
       (base*=base) %= MOD; exp >>=1; }
   return res; }
int inv(int u) { return power(u,MOD-2); }
int nAr(int n, int r) {
   int res=1;
   rep (i,r) (res*=(n-i)) %=MOD;
   return res; }
void initStirling() {
   s[1][1] = 1;
   repi (i,2,N-1) repa (j,i) s[i][j] = (s[i-1][j-1] + s[i-1][j]*j) % MOD; }
void init() {
   scanf(II,&n,&k); 
   initStirling();
}
void solve() {
   int ans=0;
   rep (j,k+1) { 
       (ans+= nAr(n,j) * power(2,n-j) % MOD * s[k][j] % MOD) %= MOD; }
   printf(IN,ans);
}