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Let and be Hilbert spaces, and let : be an unbounded operator from into . Suppose that is a closed operator and that is densely defined, that is, is dense in . Let : denote the adjoint of .
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. A Hilbert space is a special case of a Banach space. Hilbert spaces were studied beginning in the first decade of the 20th century by David Hilbert, Erhard Schmidt, and Frigyes Riesz.
The simplest example of a direct integral are the L 2 spaces associated to a (σ-finite) countably additive measure μ on a measurable space X.Somewhat more generally one can consider a separable Hilbert space H and the space of square-integrable H-valued functions
A rigged Hilbert space is a pair (H, Φ) with H a Hilbert space, Φ a dense subspace, such that Φ is given a topological vector space structure for which the inclusion map:, is continuous. [ 4 ] [ 5 ] Identifying H with its dual space H * , the adjoint to i is the map i ∗ : H = H ∗ → Φ ∗ . {\displaystyle i^{*}:H=H^{*}\to \Phi ^{*}.}
Let be a Hilbert space and an unbounded (i.e. not necessarily bounded) linear operator with a dense domain . This condition holds automatically when H {\displaystyle H} is finite-dimensional since Dom A = H {\displaystyle \operatorname {Dom} A=H} for every linear operator on a finite-dimensional space.
The space of bounded linear operators B(X) on a Banach space X is an example of a unital Banach algebra. Since the definition of the spectrum does not mention any properties of B ( X ) except those that any such algebra has, the notion of a spectrum may be generalised to this context by using the same definition verbatim.
A feature map is a map :, where is a Hilbert space which we will call the feature space. The first sections presented the connection between bounded/continuous evaluation functions, positive definite functions, and integral operators and in this section we provide another representation of the RKHS in terms of feature maps.
Hilbert's axioms, a modern axiomatization of Euclidean geometry Hilbert space , a space in many ways resembling a Euclidean space, but in important instances infinite-dimensional Hilbert metric , a metric that makes a bounded convex subset of a Euclidean space into an unbounded metric space