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Taras Banakh
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The Golomb space $(\mathbb N,\tau)$ (thea "universal" counterexample to many questions), also gives a counterexample with $|\mathbb N|=\aleph_0$ and $|\tau|=\mathfrak c$.

I recall that the Golomb space is the set $\mathbb N$ of natural numbers endowed with the topology $\tau$ generated by the base consisting of the arithmetic progressions $a+\mathbb N_0b=\{a+nb:n\ge 0\}$ where $a,b$ are relatively prime.

It is well-known that the Golomb space is connected and Hausdorff. Since it contains a countable disjoint family of open sets (like any infinite Hausdorff space), its topology has cardinality $\mathfrak c\le|\tau|\le|\mathcal P(\mathbb N)|=\mathfrak c$.

In place of the Golomb space one can take any other countable Hausdorff connected space.

Such spaces have appeared in other questions of Dominic van der Zypen:

Is there a connected $T_2$-topology on $\mathbb{Q}$ that is coarser than the Euclidean one?

Is $\mathbb{Q}$ the continuous image of a Golomb-like space, or vice versa?

Cardinality of a set of countable connected Hausdorff spaces

Continuous self-maps in the Golomb space that are neither increasing nor decreasing

The Golomb space $(\mathbb N,\tau)$ (the "universal" counterexample to many questions), also gives a counterexample with $|\mathbb N|=\aleph_0$ and $|\tau|=\mathfrak c$.

I recall that the Golomb space is the set $\mathbb N$ of natural numbers endowed with the topology $\tau$ generated by the base consisting of the arithmetic progressions $a+\mathbb N_0b=\{a+nb:n\ge 0\}$ where $a,b$ are relatively prime.

It is well-known that the Golomb space is connected and Hausdorff. Since it contains a countable disjoint family of open sets (like any infinite Hausdorff space), its topology has cardinality $\mathfrak c\le|\tau|\le|\mathcal P(\mathbb N)|=\mathfrak c$.

In place of the Golomb space one can take any other countable Hausdorff connected space.

Such spaces have appeared in other questions of Dominic van der Zypen:

Is there a connected $T_2$-topology on $\mathbb{Q}$ that is coarser than the Euclidean one?

Is $\mathbb{Q}$ the continuous image of a Golomb-like space, or vice versa?

Cardinality of a set of countable connected Hausdorff spaces

Continuous self-maps in the Golomb space that are neither increasing nor decreasing

The Golomb space $(\mathbb N,\tau)$ (a "universal" counterexample to many questions), also gives a counterexample with $|\mathbb N|=\aleph_0$ and $|\tau|=\mathfrak c$.

I recall that the Golomb space is the set $\mathbb N$ of natural numbers endowed with the topology $\tau$ generated by the base consisting of the arithmetic progressions $a+\mathbb N_0b=\{a+nb:n\ge 0\}$ where $a,b$ are relatively prime.

It is well-known that the Golomb space is connected and Hausdorff. Since it contains a countable disjoint family of open sets (like any infinite Hausdorff space), its topology has cardinality $\mathfrak c\le|\tau|\le|\mathcal P(\mathbb N)|=\mathfrak c$.

In place of the Golomb space one can take any other countable Hausdorff connected space.

Such spaces have appeared in other questions of Dominic van der Zypen:

Is there a connected $T_2$-topology on $\mathbb{Q}$ that is coarser than the Euclidean one?

Is $\mathbb{Q}$ the continuous image of a Golomb-like space, or vice versa?

Cardinality of a set of countable connected Hausdorff spaces

Continuous self-maps in the Golomb space that are neither increasing nor decreasing

Source Link
Taras Banakh
  • 42k
  • 3
  • 74
  • 184

The Golomb space $(\mathbb N,\tau)$ (the "universal" counterexample to many questions), also gives a counterexample with $|\mathbb N|=\aleph_0$ and $|\tau|=\mathfrak c$.

I recall that the Golomb space is the set $\mathbb N$ of natural numbers endowed with the topology $\tau$ generated by the base consisting of the arithmetic progressions $a+\mathbb N_0b=\{a+nb:n\ge 0\}$ where $a,b$ are relatively prime.

It is well-known that the Golomb space is connected and Hausdorff. Since it contains a countable disjoint family of open sets (like any infinite Hausdorff space), its topology has cardinality $\mathfrak c\le|\tau|\le|\mathcal P(\mathbb N)|=\mathfrak c$.

In place of the Golomb space one can take any other countable Hausdorff connected space.

Such spaces have appeared in other questions of Dominic van der Zypen:

Is there a connected $T_2$-topology on $\mathbb{Q}$ that is coarser than the Euclidean one?

Is $\mathbb{Q}$ the continuous image of a Golomb-like space, or vice versa?

Cardinality of a set of countable connected Hausdorff spaces

Continuous self-maps in the Golomb space that are neither increasing nor decreasing