# A Combinatorical Proof

Prove the following combinatorial:

The number of ways of placing $$n$$ labeled balls into $$n$$ indistinguishable boxes is equal to

$\dfrac\sum_{k=0}^{\infty} \dfrac {(k+1)^{n-1}}{k!}}{e$ Note by Sharky Kesa
4 years, 11 months ago

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What you are counting is the Bell numbers, and your result is known as Dobinski's formula.

(By the way, this is not a bijection. A bijection is a 1-1 correspondence between two sets. What you want to prove is that a certain combinatorial formula is true.)

- 4 years, 11 months ago

OK, thanks! However, I thought that a bijection could be constructed between these two by changing the summation.

- 4 years, 11 months ago

Okay so I have a solution for putting $n$ balls in exactly $k$ boxes:

Define a function that takes an ordered pair of naturals to a nonnegative integer:

$f(n,k)=\begin{cases} 1 & k=1 \\ 1 & n=k \\0 & k>n\\kf(n-1,k)+f(n-1,k-1) & \text{for other cases}\end{cases}$

This function should satisfy it if my LaTeX is correct.

Now for putting $n$ balls in $n$ boxes, just do

$\displaystyle \sum_{x=1}^{n}f(n,x)$

- 4 years, 11 months ago

Now you just need to biject this summation to the other summation.

- 4 years, 11 months ago

I looked it up and it I haven't seen any solutions which use a combinatorial bijection.

- 4 years, 11 months ago

Can you think of any other solution tghen? It might help to create a bijection.

- 4 years, 11 months ago

Are you sure that's correct? For example when $n=1$ we have $1$ way of putting a ball in a box, but the sum indicates that there are a few more than 1 way; for $k=0$ that term alone is already $1$.

- 4 years, 11 months ago

I forgot to put a denominator in the sum but it has been rectified.

- 4 years, 11 months ago