Rigged Hilbert space

In mathematics, a rigged Hilbert space (Gelfand triple, nested Hilbert space, equipped Hilbert space) is a construction designed to link the distribution and square-integrable aspects of functional analysis. Such spaces were introduced to study spectral theory in the broad sense. They can bring together the 'bound state' (eigenvector) and 'continuous spectrum', in one place.

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Motivation

Since a function such as the canonical homomorphism of the real line into the complex plane

x \mapsto e^{ix} \quad ,

which is in an obvious sense an eigenvector of the differential operator

-i\frac{d}{dx}

on the real line R, is not square-integrable for the usual Borel measure on R, this requires some way of stepping outside the strict confines of the Hilbert space theory. This was supplied by the apparatus of Schwartz distributions, and a generalized eigenfunction theory was developed in the years after 1950.

Functional analysis approach

The concept of rigged Hilbert space places this idea in abstract functional-analytic framework. Formally, a rigged Hilbert space consists of a Hilbert space H, together with a subspace Φ which carries a finer topology, that is one for which the natural inclusion

\Phi \subseteq H

is continuous. It is no loss to assume that Φ is dense in H for the Hilbert norm. We consider the inclusion of dual spaces H* in Φ*. The latter, dual to Φ in its 'test function' topology, is realised as a space of distributions or generalised functions of some sort, and the linear functionals on the subspace Φ of type

\phi\mapsto\langle v,\phi\rangle

for v in H are faithfully represented as distributions (because we assume Φ dense).

Now by applying the Riesz representation theorem we can identify H* with H. Therefore the definition of rigged Hilbert space is in terms of a sandwich:

\Phi \subseteq H \subseteq \Phi^*.

The most significant examples are for which Φ is a nuclear space; this comment is an abstract expression of the idea that Φ consists of test functions and Φ* of the corresponding distributions.

Formal definition (Gelfand triple)

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 i is continuous.

Identifying H with its dual space H*, the adjoint to i is the map

i^*:H=H^*\to\Phi^*.

The duality pairing between Φ and Φ* has to be compatible with the inner product on H, in the sense that:

\langle u, v\rangle_{\Phi\times\Phi^*} = (u, v)_H

whenever u\in\Phi\subset H and v \in H=H^* \subset \Phi^*.

The specific triple (\Phi,\,\,H,\,\,\Phi^*) is often named the "Gelfand triple" (after the mathematician Israel Gelfand).

Note that even though Φ is isomorphic to Φ* if Φ is a Hilbert space in its own right, this isomorphism is not the same as the composition of the inclusion i with its adjoint i*

i^* i:\Phi\subset H=H^*\to\Phi^*.

References