Find the formula for the following sum of binomial coefficients: $ \sum_{m\ge 0} (-1)^m {\binom{n}{m} }^3 .$ Could you find the formula for $\sum\limits_{m\ge 0} (-1)^m{\binom{n}{m}}^4$?
A sum involving powers of binomial coefficients.
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0$n$ is any fixed positive integer. – 2012-04-18
2 Answers
Your sum is a special case of Dixon's well-poised sum
$\begin{align*} \sum_{k=0}^\infty (-1)^k \binom{n}{k}^3&={}_3 F_2\left({{-n,-n,-n}\atop{1,1}}\mid 1\right)\\ &=\frac{\Gamma\left(1-\frac{n}{2}\right)\Gamma \left(\frac{3 n}{2}+1\right)}{n!\Gamma(1-n)\Gamma\left(\frac{n}{2}+1\right)^2}=\frac{\cos\left(\frac{\pi n}{2}\right)\left(\frac{3n}{2}\right)!}{\left(\left(\frac{n}{2}\right)!\right)^3} \end{align*}$
Generally,
$\sum_{k=0}^\infty (-1)^k \binom{n}{k}^p={}_p F_{p-1}\left({{-n,\cdots,-n}\atop{1,\cdots,1}}\mid (-1)^{p+1}\right)$
since $\dbinom{n}{k}=\dfrac{(-1)^k (-n)_k}{k!}$, and $(1)_k=k!$ and thus
$\sum_{k=0}^\infty (-1)^k \binom{n}{k}^p=\sum_{k=0}^\infty (-1)^k \left(\frac{(-1)^k (-n)_k}{k!}\right)^p=\sum_{k=0}^\infty ((-1)^{p+1})^k \frac{((-n)_k)^p}{((1)_k)^{p-1} k!}$
and the hypergeometric form is now noticeable.
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0How about ∑m=0n(−1)m(nm)p when p is real non-integer between 0 and 2? Are there any results like Dixon's well-poised sum – 2012-09-22
Binomial coefficient sums are mechanical via Gosper's algorithm and extensions.
Wolfram Alpha gives $\sum_{m=0}^{\infty} (-1)^m \binom{n}{m}^3 = \frac{(1-n)^\overline{n} (1+\frac{n}{2})^\overline{n}}{n! (1-\frac{n}{2})^\overline{n}}$ (where I've used slightly different notation for the Pochhammer symbol).
Similarly $\sum_{m=0}^{\infty} (-1)^m \binom{n}{m}^4 = {}_4F_3(-n, -n, -n, -n;1, 1, 1;-1)$ is hypergeometric-summable.
Checking for $k=5$ and $k=6$ leads to the hypothesis that $\sum_{m=0}^{\infty} (-1)^m \binom{n}{m}^k = {}_kF_{k-1}(-n, \ldots, -n;1, \ldots, 1;-1^{k-1})$ but I don't have time now to try to prove this.
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1"I don't have time now to try to prove this." - the proof is quite short; see my answer. – 2012-04-18