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In topology, a second-countable space, also called a completely separable space, is a topological space whose topology has a countable base.More explicitly, a topological space is second-countable if there exists some countable collection = {} = of open subsets of such that any open subset of can be written as a union of elements of some subfamily of .
Conversely, a metrizable space is separable if and only if it is second countable, which is the case if and only if it is Lindelöf. To further compare these two properties: An arbitrary subspace of a second-countable space is second countable; subspaces of separable spaces need not be separable (see below).
sequential space: a set is closed if and only if every convergent sequence in the set has its limit point in the set; first-countable space: every point has a countable neighbourhood basis (local base) second-countable space: the topology has a countable base; separable space: there exists a countable dense subset
The σ-locally finite notion is a key ingredient in the Nagata–Smirnov metrization theorem, which states that a topological space is metrizable if and only if it is regular, Hausdorff, and has a σ-locally finite base. [9] [10] In a Lindelöf space, in particular in a second-countable space, every σ-locally finite collection of sets is ...
Let be a countable basis of .Consider an open cover, =.To get prepared for the following deduction, we define two sets for convenience, := {:}, ′:=. A straight-forward but essential observation is that, = which is from the definition of base. [1]
The Fisher Space Pen was not commissioned by NASA at a cost of millions of dollars, while the Soviets used pencils. It was independently developed by Paul C. Fisher, founder of the Fisher Pen Company, with $1 million of his own funds. [19] NASA tested and approved the pen for space use, then purchased 400 pens at $6 per pen. [20]
Counterexamples in Topology (1970, 2nd ed. 1978) is a book on mathematics by topologists Lynn Steen and J. Arthur Seebach, Jr.. In the process of working on problems like the metrization problem, topologists (including Steen and Seebach) have defined a wide variety of topological properties.
An isomorphism between two spaces is defined as a one-to-one correspondence between the points of the first space and the points of the second space, that preserves all relations stipulated according to the first level. Mutually isomorphic spaces are thought of as copies of a single space. If one of them belongs to a given species then they all do.