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2h
awarded  Favorite Question
2d
comment Prove that A is invertible if $A^2 - 4A -7I = 0$.
Not in characteristic $7$, though.
2d
comment Find an arbitrary power of a lower triangular matrix of size $3\times 3$
Inducción, José!
May
24
comment Aut(G) is abelian
I assume by Zentrum you mean the center?
May
24
comment $G$ a group and $H$,$K$ subgroups, $kHk^{-1} \subseteq H \implies kHk^{-1} = H$?
Yes, yes. Merci, @Berci.
May
24
revised $G$ a group and $H$,$K$ subgroups, $kHk^{-1} \subseteq H \implies kHk^{-1} = H$?
edited body
May
23
answered $G$ a group and $H$,$K$ subgroups, $kHk^{-1} \subseteq H \implies kHk^{-1} = H$?
May
21
awarded  Famous Question
May
19
comment How to prove that $C\cdot\aleph_0=C$
You should clarify the nature of $f$. People might think it is a bijection.
May
17
awarded  Popular Question
May
17
comment Show a subset of $[0,1]^2$ has content $0$
You can try to show that your set is Jordan measurable, and then calculate it's measure which you know is 0.
May
17
comment Show a subset of $[0,1]^2$ has content $0$
Lebesgue measurable doesn't imply Jordan measurable.
May
14
comment Find image and kernel of $\varphi: \mathbb{Z}[x] \to \mathbb{C}$ given by $x \mapsto i$
Note that a polynomial $p(X)$ goes to $p(i)$. You should convince yourself the kernel is $(X^2+1)$, and the image is $\Bbb Z[i]$, which gives the isomorphism $\Bbb Z[X]/(X^2+1)\simeq \Bbb Z[i]$.
May
14
comment Let $X\sim\mathrm{Exp}(1),Y\sim\mathrm{Exp}(2)$ independent random variables. Let $Z = \min(X, Y)$. Calculate $E(Z^2)$.
The notation is standard for what the OP just said.
May
13
comment Let $X\sim\mathrm{Exp}(1),Y\sim\mathrm{Exp}(2)$ independent random variables. Let $Z = \min(X, Y)$. Calculate $E(Z^2)$.
@MichaelHardy A random variable with density $f_X(t)=2e^{-2t}$.
May
13
revised Lebesgue integrable function $g$ equals characteristic function
added 276 characters in body
May
13
answered Lebesgue integrable function $g$ equals characteristic function
May
12
awarded  Necromancer
May
12
comment Hamiltonian: Derivative
Please avoid over-editing your post. Regards,
May
10
revised List all the elements of ${\mathbb F_2}[X]/(X^3+X^2+1)$.
edited title