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Jun
20
reviewed Close Convergence of the series $\sum_{n\in\mathbb N}\left(\sin\frac{1}{n^n}\cdot 2^n\cdot n!\right)$
Jun
20
reviewed Close How to find $\lim_{x\to\infty} \frac{ \int_x^1 \arctan(t^2)\, dt}{x} $
Jun
20
reviewed Close Maximum value of trigonometric expression
Jun
20
answered Combinatorial interpretation of identity: $\sum_{j=0}^b\binom{b}{j}^2\binom{n+j}{2b}=\binom{n}{b}^2$
Jun
20
answered Alternative way to count the number of solutions to the equation $x^2 + y^2 = -1$ over $\Bbb Z /p$
Jun
20
comment Recurrence $(n+2)\text{Cat}_{n+1}=(4n+2)\text{Cat}_n$ for non-crossing matchings
Understanding is its own reward.
Jun
19
reviewed Reviewed matrix inequality proof [completion of squares]
Jun
17
comment Counting the number of solutions of equation $x^2 + y^2 = 1$ over $\Bbb Z/p$
See also an example of using Jacobi sums to count points on $y^2=x^3-x$.
Jun
17
revised Counting the number of solutions of equation $x^2 + y^2 = 1$ over $\Bbb Z/p$
P.S. about Weil etc
Jun
17
answered Counting the number of solutions of equation $x^2 + y^2 = 1$ over $\Bbb Z/p$
Jun
16
awarded  Nice Answer
Jun
15
reviewed Close Is being positive and orthogonal sufficient for being identity?
Jun
15
comment Counting the number of solutions of equation $x^2 + y^2 = 1$ over $\Bbb Z/p$
1) «Rational parametrization of conics»; 2) «Jacobi sums».
Jun
15
revised $SL_2(\mathbb Z_3)/Z(SL_2(\mathbb Z_3)) \cong \mathbb A_4$
edited tags
Jun
15
comment $SL_2(\mathbb Z_3)/Z(SL_2(\mathbb Z_3)) \cong \mathbb A_4$
From geometric POV the homomorphism to $A_4$ comes from the action of $SL_2(\mathbb Z_3)$ on 4-element set $P^1(\mathbb Z_3)$ (by fractional-linear transformations, if you will). Cf. $PSL_2(\mathbb F_4)\cong A_5$ etc.
Jun
7
comment Proof of $\sum_{n=1}^\infty \frac{1}{n^4 \binom{2n}{n}}=\frac{17\pi^4}{3240}$
related: math.stackexchange.com/q/144985
Jun
4
comment Why universal G-bundles are contractible?
If $B$ represents the functor «$X$ → principal $G$-bundles on $X$», then $E$ represents the functor «$X$ → principal $G$-bundles on $X$ with a fixed section». This functor is trivial, so $E$ is contractible.
May
14
comment Expected number of returns by time n in a symmetric 1-d random walk?
possible duplicate of Expected number of returns to zero in a symmetric random walk - closed form
May
13
comment Proving that $SL_2(\mathbb{Z}_5) / \{\pm I\}\simeq A_5$
cf. math.stackexchange.com/q/93762/152
May
6
comment Combinatorial definition of Hall–Littlewood polynomials (sum over SSYT?)
...but extracting something explicit for HL polynomials from that doesn't look like an easy task...