Closed form of $\sum_{k=1}^{2015}(-1)^{k(k+1)/2}k$
134610
Find the sum of finite series $S=\sum_{k=1}^{2015}{(-1)^{\frac{k(k+1)}{2}}}k$
151757
Simplify $\sum_{i=0}^{N/2}{N-i\choose i}$
36083
If $k > 1$, then $\frac1{(k - 1)^2}-\frac1{(k + 1)^2}=\frac{4k}{(k^2 - 1)^2}$, hence simplify $\sum\limits_{k = 2}^n\frac k{(k^2 - 1)^2}$
45862
Nested sum of binomial coefficient in closed form
36436
Proving $\sum_{k=1}^n k k!=(n+1)!-1$
127806
How to find the sum of the series $\sum_{k=2}^\infty \frac{1}{k^2-1}$?
107976
Find a closed formula for $\sum_{n=1}^\infty nx^{n-1}$
136652
Closed form expression for the product $\prod\limits_{k=1}^{n}\left(1 - \frac{1}{ak}\right)$
19633
Closed form for a pair of continued fractions
39659
Is there a closed formula for this summation?
145900
Proof of a double summation reduction identity
5272
Proving that $\sum_{k=0}^{n}\frac{(-1)^k}{{n\choose k}}=[1+(-1)^n] \frac{n+1}{n+2}.$
158616
A closed form for the sum $S = \frac {2}{3+1} + \frac {2^2}{3^2+1} + \cdots + \frac {2^{n+1}}{3^{2^n}+1}$ is ...
121617
Closed form of $\sum_{n=0}^{\infty} \frac{\cos(\alpha \, n)}{n!}$
142985
Finding sum of series $\sum^{\infty}_{k=0}\frac{(k+1)(k+2)}{2^k}$
138542
Evaluating sums question $\sum_{k=2}^n \frac{1}{k^2-1}$
153226
Proof of $\sum_{k = 1}^{n} \frac{1}{k^{2}} < 2 - \frac{1}{n}$