18,730 reputation
11229
bio website at.yorku.ca/cgi-bin/bbqa
location Netherlands
age 44
visits member for 3 years, 7 months
seen 10 hours ago

Got a PhD in topology in 98, now a cryptographer.


10h
answered How many reflexive binary relations there are on a finite countable set?
1d
answered First countable spaces, enumerable neighborhood base system
1d
comment Show that $(x_1\times\dots\times x_k)\times x_{k+1}=x_1\times\dots\times x_{k+1}$
You should define the right hand side too, if you want to be formal. By recursion, I imagine. Then the proof by induction should be clear.
1d
comment Closed and Connected subgroups of $\mathbb{R}^n$
Let us continue this discussion in chat.
1d
comment Closed and Connected subgroups of $\mathbb{R}^n$
You still have to prove that!
1d
comment Closed and Connected subgroups of $\mathbb{R}^n$
@PraphullaKoushik Not just any copy, a linear subspace copy...
1d
comment Closed and Connected subgroups of $\mathbb{R}^n$
Martini gave the hint: linear subspaces. Like the line $x=y$ in the plane, etc. And strictly speaking, $\mathbb{R}$ is not even a subset of $\mathbb{R}^2$...
1d
comment Closed and Connected subgroups of $\mathbb{R}^n$
For the singletons case just note that any subgroup must contain $0$, and so can only contain $0$.
1d
comment Any polynomial function is continuous - what about a constant function?
$f^{-1}[U] = \{x \in X: f(x) \in U \}$ by definition. It is allowed to be empty or $X$ or anything in between. The definition of continuity just says then when $U$ is open, so must the set $f^{-1}[U]$ be.
1d
comment The ambiguity of set theory language
As always in language, context is everything.
1d
answered Sequentially compact space
1d
comment Formally show that the set of continuous functions is not measurable
You do believe/see that the statement about Borel sets in $\mathbb{R}^{\mathbb{R}}$ is true?
1d
revised Formally show that the set of continuous functions is not measurable
edited body
1d
answered Distributing Set Intersections Over an Intersection
Jul
7
answered Expansion of $(1+px+qx^2)^8\equiv 1+8x+52x^2+kx^3$…
Jul
7
revised Show that a map with some properties is closed
deleted 1 character in body
Jul
7
answered Show that a map with some properties is closed
Jul
6
comment Finite $T_0$ space
@YashChandra It's a standard partial order on $T_0$ spaces, called the specialisation partial order. So it's an old idea.
Jul
6
comment A generalization of the Arhangelskii Theorem
I agree it would be better on mathoverflow.
Jul
6
answered Show that a map with some properties is closed