Hilbert cube

In mathematics, the Hilbert cube, named after David Hilbert, is a topological space that provides an instructive example of some ideas in topology. Furthermore, many interesting topological spaces can be embedded in the Hilbert cube; that is, can be viewed as subspaces of the Hilbert cube (see below). The Hilbert cube is best defined as the topological product of the intervals for That is, it is a cuboid of countably infinite dimension, where the lengths of the edges in each orthogonal direction form the sequence The Hilbert cube is homeomorphic to the product of countably infinitely many copies of the unit interval In other words, it is topologically indistinguishable from the unit cube of countably infinite dimension. Some authors use the term "Hilbert cube" to mean this Cartesian product instead of the product of the . If a point in the Hilbert cube is specified by a sequence with then a homeomorphism to the infinite dimensional unit cube is given by It is sometimes convenient to think of the Hilbert cube as a metric space, indeed as a specific subset of a separable Hilbert space (that is, a Hilbert space with a countably infinite Hilbert basis). For these purposes, it is best not to think of it as a product of copies of but instead as as stated above, for topological properties, this makes no difference. That is, an element of the Hilbert cube is an infinite sequence that satisfies Any such sequence belongs to the Hilbert space so the Hilbert cube inherits a metric from there. One can show that the topology induced by the metric is the same as the product topology in the above definition. As a product of compact Hausdorff spaces, the Hilbert cube is itself a compact Hausdorff space as a result of the Tychonoff theorem. The compactness of the Hilbert cube can also be proved without the axiom of choice by constructing a continuous function from the usual Cantor set onto the Hilbert cube. In no point has a compact neighbourhood (thus, is not locally compact). One might expect that all of the compact subsets of are finite-dimensional.

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Second-countable space

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 . A second-countable space is said to satisfy the second axiom of countability. Like other countability axioms, the property of being second-countable restricts the number of open sets that a space can have.

Hilbert space

In mathematics, Hilbert spaces (named after David Hilbert) allow the methods of linear algebra and calculus to be generalized from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise naturally and frequently in mathematics and physics, typically as function spaces. Formally, a Hilbert space is a vector space equipped with an inner product that induces a distance function for which the space is a complete metric space.

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