Grigory M
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 Jan12 reviewed Close Is $\bigl(\sum {{x^n}\over{n!}} \bigr) \bigl(\sum {{y^n}\over{n!}} \bigr) = \bigl(\sum {{(x+y)^n}\over{n!}}\bigr)$ generalizable for series? Jan12 reviewed Close How are contest problems designed? Jan11 answered A three variable binomial coefficient identity Jan11 reviewed Leave Open Definition of abs() function Jan11 reviewed Looks OK Find the limit of $\lim_{n\rightarrow\infty}(\frac{1}{2}+\frac{3}{2^2}+…+\frac{2n-1}{2^n})$ Jan10 answered Combinatorial Identity Jan10 answered Finite summation with binomial coefficients, $\sum (-1)^k\binom{r}{k} \binom{k/2}{q}$ Jan10 reviewed No Action Needed complex coordinates of perpendicular chords on unit circle Jan10 reviewed Looks OK Fundamental weights of $A_n$ Jan10 reviewed Close For the birthday problem, what if a person was chosen beforehand? Jan10 revised The smallest $n> 0$ with the nonzero $n$th Stiefel-Whitney class is a power of 2 when total Stiefel-Whitney class is not trivial. edited tags Jan10 comment Are Exponential and Trigonometric Functions the Only Non-Trivial Solutions to $F'(x)=F(x+a)$? related: math.stackexchange.com/q/199691 Jan9 revised Finite summation with binomial coefficients, $\sum (-1)^k\binom{r}{k} \binom{k/2}{q}$ more descriptive title Jan9 comment Finite summation with binomial coefficients, $\sum (-1)^k\binom{r}{k} \binom{k/2}{q}$ off the top of my head, something like math.stackexchange.com/a/609202 should work (but finding a bijective proof would be, perhaps, more challenging) Jan9 comment Finite summation with binomial coefficients, $\sum (-1)^k\binom{r}{k} \binom{k/2}{q}$ @Marko tomorrow (or later) — maybe; if you have time now — please just go ahead Jan9 reviewed Looks OK Why schemes are $(X,\mathcal O_X)$ rather than $(\mathcal O_X,X)$ or $\{X,\mathcal O_X\}$ Jan9 comment A three variable binomial coefficient identity Thank you! I'm awarding the bounty now — and will try to understand the proof later. Jan9 reviewed Close Does $\sum_{n = 2}^{\infty} \frac{\sqrt{n + 1}}{n(n-1)}$ converge or diverge? Jan9 comment A three variable binomial coefficient identity I've also asked a (different but) related question @ MO Jan9 reviewed Leave Closed How derivative relates to roots of original function