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Jul
2
awarded  Curious
May
24
revised Does every function with $f_x,f_y>0,f_{xx},f_{yy}<0$ with particular condition have to satisfy $f_{xy}/f_{xx} = -x/y$?
deleted 3 characters in body
May
24
comment Does every function with $f_x,f_y>0,f_{xx},f_{yy}<0$ with particular condition have to satisfy $f_{xy}/f_{xx} = -x/y$?
When you say $\infty$, it is $+\infty$ or $-\infty$?
May
21
answered Inverse of a sum of positive definite matrices
May
20
answered Proving $A+2B+3C+4D < 2.5$ with given conditions
May
20
comment If $f'(x) = 0$ for all $x \in \mathbb{Q}$, is $f$ constant?
You may want to consider the fundamental theorem of Lebesgue integral calculus, which requires $f'(x)=0$ almost everywhere to get to your result. However, $\mathbb{Q}$ is far away from almost everywhere..
Feb
12
awarded  Nice Answer
Jan
29
awarded  Yearling
Sep
12
accepted Maximal abelian subgroups in a $p$-group are always normal?
Sep
12
comment Maximal abelian subgroups in a $p$-group are always normal?
thanks, I didn't realise it was so simple.
Sep
11
comment Maximal abelian subgroups in a $p$-group are always normal?
@DonAntonio, yeah, I am assuming the $p$-group is finite. But I am talking about maximal among abelian subgroups, not just maximal among any subgroups.
Sep
11
asked Maximal abelian subgroups in a $p$-group are always normal?
Aug
5
comment $ \frac {dy}{dx} \sin y = (1-x\cos y)\cos y$
Hint: $\sin ydy=-d\cos y$.
Jul
31
asked How to choose the adjacency set
Jul
22
comment Are these 2 graphs isomorphic?
@MarkMcClure, do you know this kind of animation is available by tex coding or not?
Jul
22
answered Group Theory Normal Subgroups
Jul
21
comment Solving $x^2-7[x]+5=0.$ to find values of $x$.
Hint: find the intersection of $y=x^2+5$ and $y=7[x]$.
Jul
15
comment Splitting an electricity bill
I suggest to assume every equal length period consumes the same amount of electricity. We know there are a lot of electric facilities consume the same amount of electricity regardless of the number of tenancy. For example, fridge, tv, heater, etc. More people just means more usage on the light bulbs in their own rooms, maybe?
Jul
9
comment What is the structure of the Coxeter groups of type $\text{D}_n$
@JyrkiLahtonen, sorry, I still don't quite get it. For example, where $\sigma_2\circ\sigma_n$ is mapped to? And why $|ker f|=2^{n-1}$ but not $2^{n\choose 2}$?
Jul
9
comment What is the structure of the Coxeter groups of type $\text{D}_n$
@TobiasKildetoft, by the base group I meant $\mathbb{Z}_2^{n-1}$.