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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.

© 2013-2019 by Boris Haase

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