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While looking for a closed form of a expression I worked myself to a formula that resembles the Vandermonde convolution, but is summed over even binomial coefficients only.

$\sum_{k=0}^n\sum_{l=0}^n{{2k+2l}\choose{2l}}{{4n-2k-2l}\choose{2n-2l}}$

I'm at a loss as to what to do with it. I can re-write it in several ways, but the principal problem remains. Is there a known technique to attack such sums? Thanks.


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1 Answer 1

up vote 15 down vote accepted

We have $$\sum_l\binom{a+l}lx^l=\frac1{(1-x)^{a+1}},$$ hence the generating function for the even terms of the sequence is $$\sum_l\binom{a+2l}{2l}x^{2l}=\frac12\left(\frac1{(1-x)^{a+1}}+\frac1{(1+x)^{a+1}}\right).$$ Consequently, \begin{multline*}\sum_l\binom{a+2l}{2l}\binom{b+2(n-l)}{2(n-l)}=\\\\ [x^{2n}]\frac14\left(\frac1{(1-x)^{a+1}}+\frac1{(1+x)^{a+1}}\right)\left(\frac1{(1-x)^{b+1}}+\frac1{(1+x)^{b+1}}\right),\end{multline*} where $[x^n]f$ denotes the $n$th coefficient of the power series for $f$. Plugging in the actual values for $a$ and $b$ and summing over $k$ gives \begin{align*} \sum_k\sum_l&\binom{2(k+l)}{2l}\binom{2(n-k)+2(n-l)}{2(n-l)}\\\\ &=[x^{2n}]\sum_{k=0}^n\frac14\left(\frac1{(1-x)^{2k+1}}+\frac1{(1+x)^{2k+1}}\right)\left(\frac1{(1-x)^{2(n-k)+1}}+\frac1{(1+x)^{2(n-k)+1}}\right)\\\\ &=[x^{2n}]\left[\frac{n+1}4\left(\frac1{(1-x)^{2n+2}}+\frac1{(1+x)^{2n+2}}\right)+\frac{1-x^2}{8x}\left(\frac1{(1-x)^{2n+2}}-\frac1{(1+x)^{2n+2}}\right)\right]\\\\ &=\frac{2n^2+4n+1}{2n+1}\binom{4n}{2n}=(2n^2+4n+1)C_{2n}. \end{align*}

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For $n=1$, we have $\sum_k \sum_l$ ${2k+2l}\choose{2l}$ ${4n-2k-2l}\choose{2n-2l}$ $= \sum_l$ ${2l}\choose{2l}$ ${4-2l}\choose{2-2l}$ + ${2+2l}\choose{2l}$ ${2-2l}\choose{2-2l}$ = ${4}\choose{2}$ + ${2}\choose{2}$ + ${2}\choose{0}$ + ${4}\choose{2}$ = $14$. –  Zack Wolske Nov 5 '11 at 8:19
    
Thanks! In retrospective I should have figured that out myself. After fixing numerical mistakes after the second-to-last $=$ it evaluates to $\frac{n+1}{2}{4n+1 \choose 2n+1}-\frac{1}{4}{4n \choose 2n+1}+\frac{1}{4}{4n+2 \choose 2n+1}$ –  Rasto S. Nov 7 '11 at 15:41
    
I am sorry for the errors, I hope it is correct now. –  Emil Jeřábek Nov 8 '11 at 14:49
    
Yep, it's fixed. –  Zack Wolske Nov 8 '11 at 16:13
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