Homepage of Boris Haase




Previous | Next

#77: Extension Topology on 05.04.2019

In the following section, the Set Theory is presupposed.

Definition: Every irreflexive relation \(N \subseteq {A}^{2}\) defines a neighbourhood relation in \(A \subseteq X\) for the underlying set \(X\). If \((a, b) \in N\), \(a\) is called neighbour of or neighbouring to \(b\). In particular, an element \(x \in A \subseteq X\) is called neighbour of an element \(y \in A\), where \(x \ne y\) if we have for all \(z \in X\) and a mapping \(d: {X}^{2} \rightarrow \mathbb{R}_{\ge 0}\): (1) \(d(x, y) \le \text{max }\{\text{min }\{d(x, z), d(z, x)\}, \text{min }\{d(y, z), d(z, y)\}\}\) and (2) \(d(z, z) = 0\). Here \(d\) is called neighbourhood metric. Let \(P = R \cup V\) be the set of all points partitioned into actual points \(R\) and virtual points \(V\) for \(R, V \ne \emptyset = R \cap V\).

Definition: The set \(A' := R \setminus A\), where \(A \subseteq R\), is called complement of \(A\) in \(R\). When \(R\) is clear from context, it can be omitted and \(A'\) can be called the exterior of \(A\). The set \(\partial V \; (\partial A)\) consists of all points of \(V \; (A)\) that have a neighbour in \(R \; (A' \cup V)\), and is called the (inner) boundary of \(V \; (A)\). Here \('\) takes precedence over \(\partial\). When we apply \(\partial\) successively beyond that, we assume the argument to be without complement. The set \(A ° := A \setminus \partial A\) is called the interior of \(A\).

Definition: A set \(S \subseteq R \; (V)\) is said to be connected if we have for every partition of \(S\) into \(Y \cup Z\) such that \(Y, Z \ne \emptyset = Y \cap Z\): \(\partial Y' \cap \partial Z \ne \emptyset \ne \partial Z' \cap \partial Y\). \(S \subseteq R\) is moreover said to be simply connected if we have: Both \(\partial Y' \cap \partial Z \cup \partial Z' \cap \partial Y\) for every partition into connected \(Y\) and \(Z\) and \(S' \cup (\partial)V\) for \( S'\) as complement of \(S\) in \(R\) are connected for a connected (\(\partial)V\). Let \(P\) and \(R\) be simply connected.

Definition: Let \(\mathcal{P}(X) := \{A : A \subseteq X\}\) be the power set of the set \(X\). A family of sets \(\mathbb{Y} \subseteq \mathcal{P}(X)\) is called topology on \(X \subseteq R\) if every intersection and union of sets of \(\mathbb{Y}\) belongs apart from \(\emptyset\) and \(X\) to \(\mathbb{Y}\). The pair \((X, \mathbb{Y})\) is called topological space. If \(\mathbb{Y} = \mathcal{P}(X)\), the topology is called discrete. A set \(B \subseteq \mathbb{Y}\) is called a base of \(\mathbb{Y}\) if every set of \(\mathbb{Y}\) can be written as union of any number of sets of \(B\).

Examples: The base for \(\mathbb{N}, \mathbb{Z}, \mathbb{Q}, \mathbb{A}_\mathbb{R}, \mathbb{A}_\mathbb{C}, \mathbb{R}\) and \(\mathbb{C}\) is precisely each related discrete topology.

Definition: Every \(U \subseteq R\) is called neighbourhood of \(x \in R\) if \(x \in U°\). A function between two topological spaces is said to be continuous if we have for every point that can be mapped: for every neighbourhood of the image of this point there is a neighbourhood of the point whose image lies completely in the neighbourhood of the image of this point.

Definition: An \(h\)-homogeneous subset of \(R := \mathbb{R}^{m}\) for \(m \in \mathbb{N}^{*}\) is \(n\)-dimensional, where \(m \ge n \in \mathbb{N}^{*}\), if and only if it contains at least one \(n\)-cube with edge length \(h \in \mathbb{R}_{>0}\) and maximum \(n\). The definition for \(R := \mathbb{C}^{m}\) is analogous. Let be dim \({}^{(\omega)}\mathbb{C} = 2\). The set \({\mathbb{B}}_{r}(a) := \{z \in K := {}^{(\omega)}\mathbb{K}^{n} : ||z - a|| \le r\}\) for \(\mathbb{K} = \mathbb{R} \; (\mathbb{C})\) is called real (complex) (2)n-ball or briefly ball with radius \(r \in {}^{(\omega)}\mathbb{R}_{>0}\) around its centre \(a \in K\) and its boundary is called real (complex) (2)n-sphere \({\mathbb{S}}_{r}(a)\) or briefly sphere. When \(a = 0\) and \(r = 1\), we obtain the unit ball with the special case of the unit disc \(\mathbb{D}\) for \(\mathbb{K} = \mathbb{C}\) and \(n = 1\).

Examples: Every ball is simply connected and for \(r > d0\) every real \(n\)-sphere, where \(n \ge 2\), is only connected and every real 1-sphere is not connected.

Remark: The possibly misleading term of countability should not be used. The neighbouring boundary points of the conventional closed [0, 1] and the conventional open ]0, 1[ especially have not the Hausdorff property. So not every metric space can be a Hausdorff space or normal and (pre-) regular spaces are limited. The spaces \(\mathbb{C}^{n}\) and \(\mathbb{R}^{n}\) with \(n \in {}^{\omega }\mathbb{N}^{*}\) have therefore only the Fréchet topology. The situation is, however, different in (imprecise) conventional mathematics.

© 05.04.2019 by Boris Haase


Valid XHTML 1.0 • disclaimer • mail@boris-haase.de • pdf-version • bibliography • subjects • definitions • statistics • php-code • rss-feed • top