Grigory M
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 Jan3 comment What is the least number $n$, such that $n^{2015}+2015$ is prime? related: math.stackexchange.com/q/663884 Jan3 comment Show that $\mathbb{R} P^3$ is not homotopy equivalent to $\mathbb{R} P^2 \vee S^3$. ...or compute $\pi_2$ Jan3 comment How do I evaluate $\sum_{r=1}^{n} [r(r+1)(r+2)(r+3)]$? Compute answers for small $n$. Then try to guess the general answer. Then try to prove it. Jan3 comment A counter example in obstruction theory @Qiaochu This was x-posted to MO and answered there ($X=\mathbb RP^3$, $Y=\mathbb RP^2$) Jan1 comment Combinatorial definition of Hall–Littlewood polynomials (sum over SSYT?) To me GZ-patterns are as good as SSYT (the bijection is straightforward anyway) — thank you, I'll take a look! Jan1 comment Why is $\,c^2-2bcd+b^2d^2=(c-bd)^2\,$? have you tried just multiplying $c-bd$ by $c-bd$? Dec31 comment Proving $S^{4}/G$ is simply connected where $G$ is not a free group action Your space is $\Sigma(S^3/G)$ — so it's simply connected. Dec31 comment Continuous bijection from $\mathbb{R}^n$ to $\mathbb{R}^m$ See en.wikipedia.org/wiki/Invariance_of_domain (there are also numerous discussions of it here on Math.SE) Dec31 comment Alternative proof of Wedderburn's little theorem If you have a new question, please ask it by clicking the Ask Question button. Include a link to this question if it helps provide context. Dec31 comment A three variable binomial coefficient identity LHS is $\sum\binom{i+j}i\binom{n-i}j\binom{n-j}i$. So the question about $\sum\binom{n-i}j\binom{n-j}i$ looks (somewhat) related. Dec31 comment Minimum of $\newcommand{\b}[1]{\bigl(#1\bigr)} \newcommand{\f}{\frac} \b{\f3a-1}^2+\b{\f ab-1}^2+\b{\f bc-1}^2+(3c-1)^2$ @ADG could you please add some better tag(s) [than 'unknown']? Dec31 comment Polynomial with a root modulo every prime but not in $\mathbb{Q}$. Actually, I don't know (beside quadratic case). You might want to ask a separate question. Dec31 comment Polynomial with a root modulo every prime but not in $\mathbb{Q}$. Dec31 comment Evaluating $\sum\limits_{n = 1}^{\infty}{\binom{2n}{n} \frac{1}{5^n}}$ Related: math.stackexchange.com/q/69270 Dec30 comment How prove this \$\prod_{1\le i