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May
17
comment Determining if these surjections have sections
I wonder if $\pi^{-1}(A_5)$ is always isomorphic to the binary icosahedral group (and not to $S_5$)...
May
17
reviewed Close What is the largest prime number?
May
17
comment Proving $\sum_{k=1}^{n}\binom{n-1}{k-1}{\binom{n+k}{k}}^{-1}=\frac 12$ combinatorially
Since both sums have non-integral terms, it's not entirely clear what kind of combinatorial argument are you looking for...
May
17
awarded  Nice Question
May
17
reviewed Close rank($A$)=rank($A^T$)
May
16
reviewed Leave Open Area of a part of a square
May
16
reviewed Close Let $G$ be a group of order $2n$. Show that $f$ can not be surjective.
May
16
reviewed Leave Open How prove this inequality $(\sum_{k=1}^{3n}a_{k})^3\ge 27n^2\sum_{k=1}^{n}a_{k}a_{n+k}a_{2n+k}$
May
16
reviewed Looks OK Mathematics needed in the study of Quantum Physics
May
16
reviewed Close First-order homogenous linear ODE in 2 functions, with information about initial conditions
May
15
reviewed Reviewed Can we prove the formula for surface of revolution?
May
15
answered Unit or non-zero octonions form an $A_\infty$-space?
May
15
reviewed Close Beautiful Indefinite Integrals.
May
15
reviewed Close Show that $b_n > b_{n-1}$ where $\frac{a_n}{b_n}$ are the n:th harmonic number
May
15
reviewed Close The subspace sum of closed subspaces is closed
May
15
reviewed No Action Needed LimSup of Random Variable
May
15
reviewed Close Repository of functions which do not have elementary integrals
May
15
reviewed Close Combinatorial proof of $k\binom{n}{k} = n\binom{n-1}{k-1}$
May
14
reviewed Looks OK Show that two linear transformations are equal
May
14
reviewed Close Convergence of the series $\sum \frac{(-1)^{\sqrt{n}}}{n}.$