7 votes
Accepted

Can $\mathbb{R}^2$ be covered by disjoint sets homeomorphic to the union of the segments $[(0,0), (0,1)], [(0,0), (1,1)], [(0,0), (1,0)]$?

Such sets are called triods. R. L. Moore (Concerning triods in the plane and the junction points of plane continua, Proceedings of the National Academy of Sciences USA, vol. 14, 1928, pp. 85-88) ...
user avatar
  • 8,573
5 votes

Stone-Čech compactification

As for Q1, the answer is no. As remarked by Narutaka, the hyperstonean cover of $[0,1]$ does not have isolated points (Corollary 2.22) and all points of a discrete space $\Gamma$ are isolated in $\...
user avatar
  • 10.9k
5 votes

When is the Minkowski sum of weighted compact sets $w_1 B_1 + w_2 B_2 + \ldots$ (with $w \in L^1$) closed?

Another case. Let $B_1, B_2, \dots$ be compact sets in $\mathbb R^n$, all bounded by $R > 0$. Let $w_n$ be nonnegative numbers with sum $1$. The product topological space $X = B_1 \times B_2 \...
user avatar
  • 36.3k
3 votes

Open covering with bounded diameters

The answer is given by the Lebesgue's Covering Theorem (numbered as 1.8.20 in Engleking's book "Theory of dimensions: finite and infinite"): If $\mathcal F$ is a finite closed cover of the $...
user avatar
  • 32.9k
3 votes
Accepted

Distinguishing topologically weak topologies of Banach spaces

The weak topologies of the spaces $\ell_1$ and $L_1$ are not homeomorphic because of the following Theorem. Assume that $X,Y$ are two Banach spaces whose weak topologies are homeomorphic. If $X$ has ...
user avatar
  • 32.9k
2 votes

Under what general conditions is the set $S := \left\{\int_{X}v(x)\pi(x)\,\mathrm{d}P(x) \mid \pi: X \to A\right\}$ closed?

I guess that if $v$ is $P$-integrable then the answer is positive, and actually the set is compact. Indeed, what you are looking for in this case is the compactness of the Aumann integral of the ...
user avatar
  • 1,639
2 votes
Accepted

Can one explore a surface along ‘piecewise planar’ curves?

Consider the set $S=\{\,(t,t^2,t^3)\in\mathbb R^3\mid\,t\in\mathbb R\}$. Clearly $S$ is connected, and so is its complement $A=\mathbb R^3\setminus S$. Note that each plane has at most 3 points of ...
user avatar
2 votes

What is definition of branched covering?

At the top of page ten of this paper the authors write "the standard two-sheeted branched covering of the sphere by the torus, branched over four points which become the four boundary circles of ...
user avatar
  • 18.6k
1 vote

A neighborhood $Y$ of a set $X$ such that the line segment connecting any point in $Y$ and its projection to $X$ is contained in $Y$

This is to confirm the stronger claim mentioned in (1). Set $X_r=X\cap \overline{B_r(0)}$ and $X^r=X\setminus B_r(0)$ (both are closed, for any $t$). Define $$ f(t)=\min\left\{1,\sup\left\{r\colon ...
user avatar
1 vote
Accepted

Is there at least one path in the common boundary of two open sets?

The answer to the second question is yes: there is an arc containing uncountable points of $B\cap\partial C$. It is enough to prove it in the case $n=2$. Applying an affine transformation if necessary,...
user avatar
  • 3,232
1 vote

Can $\mathbb{R}^2$ be covered by disjoint sets homeomorphic to the union of the segments $[(0,0), (0,1)], [(0,0), (1,1)], [(0,0), (1,0)]$?

The question been answered in How many tacks fit in the plane?. In fact Moore proved a multidimensional version of the theorem: only countably many sets homeomorphic to the $n$-dimensional disc with ...
user avatar
1 vote

Can $\mathbb{R}^2$ be covered by disjoint sets homeomorphic to the union of the segments $[(0,0), (0,1)], [(0,0), (1,1)], [(0,0), (1,0)]$?

One sketch is as follows: For each Y-set, we can put a small open disk in the vertex, such that the three prongs divide the disk in three sectors. In each sector, we can find a rational coordinate, ...
user avatar
1 vote
Accepted

Consistency of the Hurewicz dichotomy property

The anwer is in: Tall, Franklin D.; Todorcevic, Stevo; Tokgöz, Seçil, The strength of Menger’s conjecture. In the paper they prove (among other things) that the Hurewicz dichotomy extended to all ...
user avatar
  • 803

Only top scored, non community-wiki answers of a minimum length are eligible