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Author Archives: dominiczypen
Coloring connected Hausdorff spaces
Motivation. I stumbled over the following hypergraph coloring concept when reading about an old (and open) problem by Erdos and Lovasz. Let be a hypergraph such that for all we have , and let be a set. Then a map … Continue reading
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Ihm ging ein Licht auf: Grieche entdeckte vor 2500 Jahren die Sonne
Ein Ereignis ist in diesen Sommertagen etwas in Vergessenheit geraten: Vor 2500 Jahren hat der Grieche Heliotides die Sonne entdeckt. Aus heutiger Sicht schwer zu glauben, dass diese Entdeckung eines grossen Kopfes bedurfte – aber schliesslich hatte es auch einen … Continue reading
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Homogeneous spaces generalized – and an open problem
Homogeneous spaces are topological spaces that locally look everywhere the same. Put in mathematical terms, is homogeneous if for any there is an isomorphism such that . We can generalize this by not restricting ourselves to a pair of points … Continue reading
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Two notions of continuity in the space of natural sequences
In this presentation by Martín Escardó I came across a nice concept of continuity in the set of all functions . For and we say that if and only if for all we have . Then we define to be finitely … Continue reading
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A reformulation of Hadwiger’s conjecture
Let be a finite, simple, undirected graph. Hadwiger’s conjecture states that if then is a minor of . Let’s call that statement (Hadw). We show that it is equivalent to the following statement: (Hadw’): If is not a complete graph, … Continue reading
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Accumulation without converging subsequence
“Any sequence with an accumulation point also has a subsequence converging to that point” — right? I believed this until yesterday when I stumbled over the following curious, and countable, Hausdorff space. Endow with the following topology: is given the … Continue reading
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Measuring subsets of N
Recently I tried to put a measure on the powerset of the natural numbers , but it was pointed out in a comment that the map I defined is not even finitely additive. Question: Is there a map with the … Continue reading
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