diff --git a/spaces/S000106/README.md b/spaces/S000106/README.md new file mode 100644 index 000000000..53fe22b22 --- /dev/null +++ b/spaces/S000106/README.md @@ -0,0 +1,41 @@ +--- +uid: S000106 +name: Direct limit $\mathbb R^\infty$ of Euclidean spaces $\mathbb R^n$ +refs: + - wikipedia: Direct_limit + name: Direct limit on Wikipedia + - mathse: 3961052 + name: Answer to "Is the weak topology on $\mathbb{R}^{\infty}$ the same as the box topology?" + - mathse: 5012784 + name: Answer to "Is $\ell^\infty$ with box topology connected?" + - zb: "0298.57008" + name: Characteristic classes (Milnor-Stasheff) + - zb: "0307.55015" + name: Fibre bundles. 2nd ed. (Husemoller) + - zb: "1280.54001" + name: Geometric aspects of general topology. (Sakai) + - wikipedia: Fréchet–Urysohn_space + name: Fréchet–Urysohn space on Wikipedia +--- +$X$ is the subset $\mathbb{R}^\infty$ of eventually $0$ sequences in $\mathbb{R}^\omega$, +with the [final topology](https://en.wikipedia.org/wiki/Final_topology) +with respect to the standard inclusion maps $\mathbb{R}^n \hookrightarrow \mathbb{R}^\infty$, +$x \mapsto (x^1, \ldots, x^n, 0, \ldots)$. +Thus, a set $U \subseteq \mathbb{R}^\infty$ is open iff $U \cap \mathbb{R}^n$ +is open in $\mathbb{R}^n$ for each $n$, +where we identify each Euclidean space $\mathbb{R}^n$ with its image. + +The space $\mathbb{R}^\infty$ is the [direct limit](https://en.wikipedia.org/wiki/Direct_limit) +$\varinjlim \mathbb{R}^n$ of the direct system consisting of Euclidean spaces $\mathbb R^n$ +and standard inclusion maps $\mathbb{R}^n \hookrightarrow \mathbb{R}^m$, +$x \mapsto (x^1, \ldots, x^n, 0, \ldots,0)$, for each $n < m$. + +Equivalently, $\mathbb{R}^\infty \subset \mathbb{R}^\omega$ has the subspace topology, where +$\mathbb{R}^\omega$ is given the box topology; this is shown in {{mathse:3961052}}. Moreover, +it is shown in {{mathse:5012784}} that $\mathbb{R}^\infty$ is a quasi-component of the origin in +$\mathbb{R}^\omega$. Hence $\mathbb{R}^\infty$ embeds into {S107} +as a path component. + +Defined on page 62 of {{zb:0298.57008}}, on page 2 of {{zb:0307.55015}}, +on page 56 of {{zb:1280.54001}}, and on {{wikipedia:Fréchet–Urysohn_space}} +under Direct limit of finite-dimensional Euclidean spaces. diff --git a/spaces/S000106/properties/P000238.md b/spaces/S000106/properties/P000238.md new file mode 100644 index 000000000..395bacf5f --- /dev/null +++ b/spaces/S000106/properties/P000238.md @@ -0,0 +1,16 @@ +--- +space: S000106 +property: P000238 +value: true +refs: + - zb: "1280.54001" + name: Geometric aspects of general topology. (Sakai) +--- + +$\mathbb{R}^\infty$ with the natural scalar multiplication and addition operations is a vector space +over $\mathbb{R}$. It remains to argue each operation is continuous. + +For each $n$, the scalar multiplication function $\mathbb{R} \times \mathbb{R}^\infty \to \mathbb{R}^\infty$ restricted to $\mathbb{R} \times \mathbb{R}^n \to \mathbb{R}^n \hookrightarrow \mathbb{R}^\infty$ is continuous, since the first map identifies with the usual scalar multiplication of $n$-dimensional Euclidean space. Since {S25|P130}, Proposition 2.8.3 of {{zb:1280.54001}} implies $\mathbb{R} \times \mathbb{R}^\infty \cong \varinjlim (\mathbb{R} \times \mathbb{R}^n)$. This means that $\mathbb{R} \times \mathbb{R}^\infty$ has the final topology with respect to the inclusions +$\mathbb{R} \times \mathbb{R}^n \hookrightarrow \mathbb{R} \times \mathbb{R}^\infty$. By the universal property of the final topology, it follows that $\mathbb{R} \times \mathbb{R}^\infty \to \mathbb{R}^\infty$ is continuous. + +Since each $\mathbb{R}^n$ is locally compact, Proposition 2.8.4 of {{zb:1280.54001}} implies $\mathbb{R}^\infty \times \mathbb{R}^\infty \cong \varinjlim (\mathbb{R}^n \times \mathbb{R}^n)$. Making a similar argument as above shows that the natural addition operation on $\mathbb{R}^\infty$ is continuous. \ No newline at end of file diff --git a/spaces/S000106/properties/P000240.md b/spaces/S000106/properties/P000240.md new file mode 100644 index 000000000..d8c5fa640 --- /dev/null +++ b/spaces/S000106/properties/P000240.md @@ -0,0 +1,11 @@ +--- +space: S000106 +property: P000240 +value: true +refs: + - zb: "1280.54001" + name: Geometric aspects of general topology. (Sakai) +--- + +The chain of subspaces $\empty \subset \mathbb{R}^0 \subset \mathbb{R}^1 \cdots$ is a CW structure. + diff --git a/spaces/S000107/README.md b/spaces/S000107/README.md index 3eb8224cf..0cec522b3 100644 --- a/spaces/S000107/README.md +++ b/spaces/S000107/README.md @@ -1,6 +1,6 @@ --- uid: S000107 -name: Countable box product of reals +name: Countable box product of reals $\square^\omega\mathbb R$ aliases: - Countable boolean product of reals - Box product topology on R^ω