### Introduction to General Topology

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At an isolated point, every function is continuous.

In several contexts, the topology of a space is conveniently specified in terms of limit points. In many instances, this is accomplished by specifying when a point is the limit of a sequence , but for some spaces that are too large in some sense, one specifies also when a point is the limit of more general sets of points indexed by a directed set , known as nets. In the former case, preservation of limits is also sufficient; in the latter, a function may preserve all limits of sequences yet still fail to be continuous, and preservation of nets is a necessary and sufficient condition.

Every continuous function is sequentially continuous. If X is a first-countable space and countable choice holds, then the converse also holds: any function preserving sequential limits is continuous. In particular, if X is a metric space, sequential continuity and continuity are equivalent. For non first-countable spaces, sequential continuity might be strictly weaker than continuity. The spaces for which the two properties are equivalent are called sequential spaces. This motivates the consideration of nets instead of sequences in general topological spaces.

Continuous functions preserve limits of nets, and in fact this property characterizes continuous functions. In these terms, a function.

That is to say, given any element x of X that is in the closure of any subset A , f x belongs to the closure of f A. This is equivalent to the requirement that for all subsets A ' of X '. Then, the identity map. More generally, a continuous function. Symmetric to the concept of a continuous map is an open map , for which images of open sets are open. In fact, if an open map f has an inverse function , that inverse is continuous, and if a continuous map g has an inverse, that inverse is open.

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A bijective continuous function with continuous inverse function is called a homeomorphism. If a continuous bijection has as its domain a compact space and its codomain is Hausdorff , then it is a homeomorphism.

If S has an existing topology, f is continuous with respect to this topology if and only if the existing topology is coarser than the final topology on S. Thus the final topology can be characterized as the finest topology on S that makes f continuous. If f is surjective , this topology is canonically identified with the quotient topology under the equivalence relation defined by f. Dually, for a function f from a set S to a topological space, the initial topology on S has as open subsets A of S those subsets for which f A is open in X.

If S has an existing topology, f is continuous with respect to this topology if and only if the existing topology is finer than the initial topology on S. Thus the initial topology can be characterized as the coarsest topology on S that makes f continuous. If f is injective, this topology is canonically identified with the subspace topology of S , viewed as a subset of X.

Formally, a topological space X is called compact if each of its open covers has a finite subcover. Otherwise it is called non-compact. Explicitly, this means that for every arbitrary collection. Some branches of mathematics such as algebraic geometry , typically influenced by the French school of Bourbaki , use the term quasi-compact for the general notion, and reserve the term compact for topological spaces that are both Hausdorff and quasi-compact. A compact set is sometimes referred to as a compactum , plural compacta.

Every closed interval in R of finite length is compact.

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More is true: In R n , a set is compact if and only if it is closed and bounded. See Heine—Borel theorem. Every continuous bijection from a compact space to a Hausdorff space is necessarily a homeomorphism. Every sequence of points in a compact metric space has a convergent subsequence. Every compact finite-dimensional manifold can be embedded in some Euclidean space R n. A topological space X is said to be disconnected if it is the union of two disjoint nonempty open sets.

Otherwise, X is said to be connected. A subset of a topological space is said to be connected if it is connected under its subspace topology. Some authors exclude the empty set with its unique topology as a connected space, but this article does not follow that practice. Every interval in R is connected.

The continuous image of a connected space is connected.

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The maximal connected subsets ordered by inclusion of a nonempty topological space are called the connected components of the space. Every component is a closed subset of the original space. It follows that, in the case where their number is finite, each component is also an open subset.

However, if their number is infinite, this might not be the case; for instance, the connected components of the set of the rational numbers are the one-point sets, which are not open.

## Introduction To General Topology

A space in which all components are one-point sets is called totally disconnected. Related to this property, a space X is called totally separated if, for any two distinct elements x and y of X , there exist disjoint open neighborhoods U of x and V of y such that X is the union of U and V. Clearly any totally separated space is totally disconnected, but the converse does not hold. For example, take two copies of the rational numbers Q , and identify them at every point except zero. The resulting space, with the quotient topology, is totally disconnected.

However, by considering the two copies of zero, one sees that the space is not totally separated. In fact, it is not even Hausdorff , and the condition of being totally separated is strictly stronger than the condition of being Hausdorff. A path-component of X is an equivalence class of X under the equivalence relation , which makes x equivalent to y if there is a path from x to y.

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The space X is said to be path-connected or pathwise connected or 0-connected if there is at most one path-component, i. Again, many authors exclude the empty space. Every path-connected space is connected. However, subsets of the real line R are connected if and only if they are path-connected; these subsets are the intervals of R. Also, open subsets of R n or C n are connected if and only if they are path-connected. Additionally, connectedness and path-connectedness are the same for finite topological spaces.

The product topology is sometimes called the Tychonoff topology. In general, the product of the topologies of each X i forms a basis for what is called the box topology on X. In general, the box topology is finer than the product topology, but for finite products they coincide. Related to compactness is Tychonoff's theorem : the arbitrary product of compact spaces is compact. Many of these names have alternative meanings in some of mathematical literature, as explained on History of the separation axioms ; for example, the meanings of "normal" and "T 4 " are sometimes interchanged, similarly "regular" and "T 3 ", etc.

Many of the concepts also have several names; however, the one listed first is always least likely to be ambiguous. Most of these axioms have alternative definitions with the same meaning; the definitions given here fall into a consistent pattern that relates the various notions of separation defined in the previous section. Items related to Introduction To General Topology -. Home Z. Mamuzic - Introduction To General Topology -. Introduction To General Topology - Z. Mamuzic - Published by P. Noordhoff Publishing -, Condition: Good Hardcover.

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