Grigory M
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11,615
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 Oct 9 comment Another binomial coefficients sum Oct 8 reviewed Close If a real $2\times2$ matrix satisfies $A^4=I$, does it follow that $A^2=\pm I$? Oct 3 answered Area of hyperbolic triangle definition Oct 3 comment Area of hyperbolic triangle definition @KWSK Well, (hyperbolic) lengths of both the base and the (hyperbolic) perpedicular are manifestly invariant under hyperbolic motions. Oct 3 comment Area of hyperbolic triangle definition Well, one reason is that in hyperbolic geometry $ah_a\neq bh_b$, so this 'area' would depend on the side we choose as the base — so in a sense it's not well-defined. Oct 3 comment Area of a right angled hyperbolic triangle as function of side lengths Is $\tan(S/2)=\tanh(a/2)\tanh(b/2)$ nice enough? Oct 1 comment Combinatorial interpretation of identity: $\sum_{j=0}^b\binom{b}{j}^2\binom{n+j}{2b}=\binom{n}{b}^2$ (Re: generalization) indeed Oct 1 comment Combinatorial interpretation of identity: $\sum_{j=0}^b\binom{b}{j}^2\binom{n+j}{2b}=\binom{n}{b}^2$ @Darij, Alexander thank you — I hope I fixed the typos Oct 1 revised Combinatorial interpretation of identity: $\sum_{j=0}^b\binom{b}{j}^2\binom{n+j}{2b}=\binom{n}{b}^2$ fixing typos Sep 30 reviewed Close How many $n$-digit decimal sequences (using the digits $0 = 9$) are there in which the digits $1$, $2$ and $3$ all appear? Sep 30 reviewed Looks OK Subrings and characteristic of a ring Sep 30 reviewed Close What is the simplest proof about this differentiation property? Sep 30 reviewed Close Subrings and characteristic of a ring Sep 28 reviewed Close Inequality in a triangle: $BC > \frac 12AB$ if $∠A > ∠B$ (self-answered) Sep 28 awarded binomial-coefficients Sep 27 reviewed Close How do you calculate the absolute value of trigonometric functions? Sep 27 reviewed Looks OK How to show the following limit exist by definition? Sep 26 reviewed Looks OK Why is a horseshoe not a ring Sep 24 reviewed Close Cardinality of the reals and the power set of naturals. Sep 24 reviewed Close How to find all positive integers $x,y$ such that $x^x+y=xy^3$?