Reputation
11,475
Next tag badge:
358/400 score
65/80 answers
Badges
3 41 87
Impact
~155k people reached

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 If $A^3=0$, then $(I-A)^{-1}=I+A+A^2$
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 $x^x+y=xy^3$?