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Jan
16
reviewed Reject If $\operatorname{rank}\left( \begin{bmatrix} A &B \\ C &D \end{bmatrix}\right)=n$ Prove that $\det(AD)=\det(BC)$
Jan
15
comment Unimodality of q-binomial coefficients
Unimodality of q-binomial coefficients is a difficult theorem, proved more than 20 years after it was conjectured. If you're really interested in a proof — it's easy to google references.
Jan
15
reviewed Close If $f$ is an injection, $f(S_1 \cap S_2) = f(S_1) \cap f(S_2)$
Jan
15
reviewed Close Every continuous function $f:[0,1]\rightarrow \mathbb{R}$ is bounded above
Jan
15
reviewed Close Simplify the following fraction $\frac {2+3i}{6-i}$
Jan
15
comment Finite summation with binomial coefficients, $\sum (-1)^k\binom{r}{k} \binom{k/2}{q}$
@655321 though there is one example that is explained in detail in many books (e.g. in Enumerative Combinatorics): Lagrange inversion formula
Jan
15
comment Finite summation with binomial coefficients, $\sum (-1)^k\binom{r}{k} \binom{k/2}{q}$
@655321 I'm afraid I don't know a good reference...
Jan
15
reviewed Close Set Theory intersection and union
Jan
15
reviewed Looks OK Online tool for making graphs (vertices and edges)?
Jan
14
reviewed Approve Set theory questions about $ϵ$ and concatenation
Jan
14
reviewed No Action Needed Decomposition if matrix with strongly correlated columns
Jan
14
reviewed Leave Closed Integral $\int \frac{dx}{\left(1+x^{2014}\right)\left(1+x^{\alpha}\right)},\quad \alpha\neq 0.$
Jan
14
reviewed Reject Euler path in cube
Jan
14
comment Embedding of two-dimensional CW complexes which induces a zero homomorphism on second homotopy groups
x-posted to MO: mathoverflow.net/q/193845
Jan
14
comment When $\frac{C(n, k)}{n^{k-1}} > 1$?
Well, for $n=k!+t$ we need to compare $(1+t/k!)\cdot(1-1/(k!+t))(1-2/(k!+t))\ldots(1-(k-1)/(k!+t))$ withs 1. Shouldn't be hard (take logarithm, bound it...) — and at least genesis of $s_k$ is clear.
Jan
13
reviewed Close Show that the function $d(x, y)$ is a metric on the set $\mathbb R^2$ .
Jan
13
reviewed No Action Needed Baby Rudin: Chapter 1, Problem 6{d}. How to complete this proof?
Jan
13
reviewed Close Prove Summation to Some Number $n$
Jan
13
reviewed Approve How to calculate $\lim \limits_{x \to 0}{\frac{\sqrt{1 + x + x^2} - 1}{x}}$?
Jan
13
reviewed Close $\lim_{n \to \infty}\sqrt{2+\sqrt{2+\sqrt{2}+\sqrt{2}}}$