# Sum of Lacunary Series of Binomial Coefficients with Rising Upper Parameter

It can be shown from some rather tedious expansion (or wolframalpha!) that $$\sum_{r=1}^n \binom {2r}2=\frac 16n(n+1)(4n-1)$$

Is it possible to derive this closed form result easily using summation identities of binomial coefficients, with minimal or no algebraic expansion?

The result can also be expressed as $\displaystyle\binom {n+1}2 \frac {4n-1}3$ and resembles the result for the sum of squares. Not that it would necessarily help.

Note also that $\displaystyle\sum_{r=1}^n \binom r2=\binom {n+1}3$.

• The summation index is $r$, yet there is no term under the sum that depends on $r$. Commented Nov 21, 2016 at 18:33
• Well, you can use that $\binom{2n}{2}=4\binom{n}{2}+\binom{n}{1}$ and use some of the usual identities. You can derive that by a counting argument - you can choose $2$ numbers from $\{1,\dots,2n\}$ by either choosing a pair $2k,2k-1$, or choosing a pair not of this form, which amounts to picking a distinct pair $a,b\in\{1,2,\dots,n\}$ and then picking one of the four pairs $\{2a,2b\},\{2a-1,2b\},\{2a,2b-1\},\{2a-1,2b-1\}$. Commented Nov 21, 2016 at 18:33
• That results in $4\binom{n+1}{3}+\binom{n+1}{2}$ Commented Nov 21, 2016 at 18:38
• @Dr.MV - There is now! Thanks. Commented Nov 22, 2016 at 1:26
• @ThomasAndrews - Thanks! That's really neat. How would you derive it algebraically? If you would post your comment as a solution I'd upvote and accept it. Commented Nov 22, 2016 at 1:29

You can write:$$\binom{2n}{2}=4\binom{n}{2}+\binom{n}{1}$$ This can be seen with a counting argument. The set of pairs of distinct elements in $\{1,2,\dots,2n\}$ contains $n$ pairs of the form $\{2k-1,2k\}$. The paiselrs not of this form can be found by selecting a pair of distinct values, $a,b\in\{1,2,\dots,n\}$ and then choosing any of the four pairs of $\{2a,2b\},\{2a-1,2b\},\{2a,2b-1\},$ or $\{2a-1,2b-1\}$.

Numerically, any polynomial of degree $k$ can be written as:

$$a_k\binom{n}{k}+a_{k-1}\binom{n}{k-1}+\cdots +a_0\binom{n}{0}$$

For your example, $$\binom{2n}{2}=n(2n-1)=2n^2-n = 4\frac{n^2-n}{2}+n=4\binom{n}{2}+\binom{n}{1}.$$