Summation with binomial coefficient

by fungarwai, Jan 12, 2019, 2:04 PM

Summation with one binomial coefficient
$ \sum_{r=0}^n \binom nr = 2^{n} $ from Binomial theorem

$\sum_{r=m}^n (-1)^r \binom Nr =-(-1)^{m-1}\binom{N-1}{m-1}+(-1)^n \binom{N-1}n$

Proof

$\sum_{k=m}^{n} (-1)^k {\binom{N}{k}}^{-1}
=\frac{N+1}{N+2}\left((-1)^m{\binom{N+1}{m}}^{-1}-(-1)^{n+1}{\binom{N+1}{n+1}}^{-1}\right)$

Proof

$\sum_{N=m}^n\binom{N}{k}^{-1}=\frac{k}{k-1}\left(\binom{m-1}{k-1}^{-1}-\binom{n}{k-1}^{-1}\right)$

Proof

$\sum_{r=0}^n (-1)^r \binom{n}{r}\frac{1}{r+s}=\frac{n!(s-1)!}{(n+s)!}$

Proof

$\sum_{r=1}^n (-1)^{r-1}\binom{n}{r}\frac{1}{r}=\sum_{r=1}^n \frac{1}{r}$

Proof

$\sum_{k=0}^\infty (-1)^k \binom{m-k}{k}=\begin{cases}
1 & m\equiv 0\pmod{6}\\
1 & m\equiv 1\pmod{6}\\
0 & m\equiv 2\pmod{6}\\
-1 & m\equiv 3\pmod{6}\\
-1 & m\equiv 4\pmod{6}\\
0 & m\equiv 5\pmod{6}
\end{cases}
=\begin{cases}
(-1)^m & m\equiv 0\pmod{3}\\
-(-1)^m & m\equiv 1\pmod{3}\\
0 & m\equiv 2\pmod{3}\\
\end{cases}$

Proof

$\sum_{k=0}^\infty (-1)^k \frac{1}{m-k}\binom{m-k}{k}
=\begin{cases}
\frac{2}{m} & m\equiv 0\pmod{6}\\
\frac{1}{m} & m\equiv 1\pmod{6}\\
\frac{-1}{m} & m\equiv 2\pmod{6}\\
\frac{-2}{m} & m\equiv 3\pmod{6}\\
\frac{-1}{m} & m\equiv 4\pmod{6}\\
\frac{1}{m} & m\equiv 5\pmod{6}\\
\end{cases}
=\begin{cases}
\frac{2(-1)^m}{m} & m\equiv 0\pmod{3}\\
\frac{-(-1)^m}{m} & m\equiv 1\pmod{3}\\
\frac{-(-1)^m}{m} & m\equiv 2\pmod{3}\\
\end{cases}$

Proof

$\sum_{m=0}^{\lfloor\frac{n}{k}\rfloor}{n\choose km+r}=
\frac{1}{k} \sum_{j=0}^{k-1}  e^{\frac{-2rj\pi i}{k}}(1+e^{\frac{2j\pi i}{k}})^n$

$ F_n=\sum_{i=0}^{\infty} \binom {n-i}{i}$
Proof

$\sum^n_{i=r}{i\choose r}={n+1\choose r+1}$ which is Hockey-stick identity

Other forms

Summation with two binomial coefficient
$\sum_{i=0}^n \binom {r_1+n-1-i}{r_1-1} 
\binom {r_2+i-1}{r_2-1}=\binom {r_1+r_2+n-1}{r_1+r_2-1}$

Proof with Generating function

$\sum_{i=0}^k \binom ni \binom m{k-i}=\binom {n+m}k$ which is Vandermonde's identity

Proof with Generating function

Proof with Hypergeometric function

Example

Summation with three binomial coefficient

${\binom {n+k}k}^2=\sum_{j=0}^k {\binom kj}^2 \binom {n+2k-j}{2k}$ which is the Li Shanlan identity

Proof with Hypergeometric function

Summation with multiple binomial coefficient

$\sum_{k_{ij}} {n_1 \choose k_{11}, k_{12}, \dots, k_{1t}}
\dots {n_s \choose k_{s1}, k_{s2}, \dots, k_{st}}
={n_1+n_2+\dots+n_s \choose r_1, r_2, \dots, r_t}$

where ${n \choose n_1, n_2, \dots, n_m}
=\frac{n!}{n_1!n_2!\dots n_m!},k_{1l}+k_{2l}+\dots+k_{sl}
=r_l,l=1,\dots,t$

It is the Generalized Vandermonde's identity

Proof

$\sum_{n_i\ge m_i\atop n_1+n_2+\dots+n_k =n}
\binom{n_1}{m_1}\binom{n_2}{m_2}\dots\binom{n_k}{m_k}=
\binom{n+k-1}{\sum m_i+k-1}$

Named as Vandermonde type convolution formula in
Combinatorial identities associated with new families of the numbers and polynomials and their approximation values
(Theorem 5.4 of Section 5)

The probability of the sum of negative binomial random variables also implies that

$\sum_{n_1+n_2+\cdots+n_k=n} \binom{n_1+m_1-1}{n_1}\binom{n_2+m_2-1}{n_2}\cdots 
\binom{n_k+m_k-1}{n_k}=\binom{n+m-1}{n}$

Reference: Negative binomial distribution - sum of two random variables


Proof by recalculating number of integer solution of a linear equation

Proof by generating functions of negative binomial series

Proof by sum of negative binomial random variables

It can be further generalised as:

$\displaystyle \sum_{\sum c_in_i=n} \binom{n_1}{m_1}\binom{n_2}{m_2}\cdots 
\binom{n_k}{m_k}=[x^{n-\sum c_im_i}]\prod_{i=1}^k\frac{1}{(1-x^{c_i})^{m_i+1}}$

Proof by generating functions of negative binomial series

Example
This post has been edited 24 times. Last edited by fungarwai, Yesterday at 12:54 AM

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Simply amazing!

by cardesigner06, Apr 30, 2019, 6:41 PM

Notable algebra methods with proofs and examples

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